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40 MM Antiaircraft Gun, OP 820, 1943, is a Navy service manual for the most widely used anti-aircraft gun of WW II.

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PRELIMINARY ORDNANCE PAMPHLET No. 820
40 MM
ANTIAIRCRAFT
40MM MACHINE GUN MECHANISM
MARK 1 MARK 2 MARK 1, MOD. 1. MARK 2, MOD. 1
40MM GUN BARREL
MARK 1.
40MM SIGHT 41
MARK 3 MARK 4
DESCRIPTION AND OPERATION
Department of the Navy Bureau of Ordnance
OCTOBER 1943

 

Copy#8<br>
Change #3 in 6-25-45
See Copy #4 for change #1.
Change II entered on Page 2.

 

PRELIMINARYORDNANCE PAMPHLET No. 820

40 MM ANTIAIRCRAFT GUN


40MM MACHINE GUN MECHANISM MARK 1
40MM MACHINE GUN MECHANISM MARK 2
40MM MACHINE GUN MECHANISM MARK 1, MOD. 1.- Left
40MM MACHINE GUN MECHANISM MARK 2, MOD. 1 - Right
40MM GUN BARREL MARK 1
40MM SIGHT MARK 3
40MM SIGHT MARK 4


DESCRIPTION AND OPERATION

Department of the Navy Bureau of Ordnance

OCTOBER 1943

 

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NAVY DEPARTMENT
BUREAU OF ORDNANCE
WASHINGTON, D. C.

ORDNANCE PAMPHLET No. 820
40MM ANTIAIRCRAFT GUN
October 1943

1. Ordnance Pamphlet No. 820 provides a description of the construction and operation of the 40MM Machine Gun Mechanism, and descriptions of the gun barrel and sight. It also includes operating instructions and instructions for the care and maintenance of the gun.

2. Ordnance Pamphlet No. 820-A may be used where a less detailed pamphlet is desired.

3. This pamphlet supersedes Ordnance Data No. 3781, preliminary manual for the description and operation of the 40MM antiaircraft gun. Ordnance Data No. 3781 should be destroyed. Ballistic Data for 40MM Guns are given in Ordnance Pamphlet No. 867. This pamphlet supersedes OP. 820A, which should be destroyed.

4. Ordnance Pamphlet No. 820 is UNCLASSIFIED.

M Blandly
Rear Admiral, U.S.N.
Chief of Bureau of Ordnance
 

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PREFACE

This Ordnance Pamphlet, No. 820, applies
to the following:

40MM MACHINE GUN MECHANISMS,
MARK 1 AND MARK 2,
MARK 1, MOD. 1 AND MARK 2, MOD. 1

40MM GUN BARREL, MARK 1

40MM SIGHTS, MARK 3 AND MARK 4

Standard Navy nomenclature is used herein, and is considered preferable to that used on drawings and in previous publications relating to these mechanisms.

York Safe and Lock Co.
York, Pennsylvania

 

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This page blank.
 

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TABLE OF CONTENTS

LIST OF ILLUSTRATIONS 11
CHAPTER I
INTRODUCTION 15
CHAPTER II
MECHANISM 17
Part One-Description of Gun Mechanism 17
Slide Assembly 17
Trunnion 18
Trigger Mechanism 18
Hand Operating Mechanism 19
Top Door 21
Side Door 22
Rear Door 22
Bottom Cover 23
Extractor Release Lever 23
Breech Mechanism Assembly 24
Housing Assembly 24
Breech Block Assembly 28
Loader Assembly 30
Feeding Mechanism 31
Rammer Tray 32
Catch Levers 34
Cocking Mechanism 34
Recoil Cylinder Assembly 37
Piston Rod 37
Throttling Rod 37
Needle Valve 37
Recoil Fluid 38
 

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MECHANISM (Continued)
Barrel Assembly 38
Differences Between Mark 1 and Mark 2 Mechanisms 38
Slide Assembly 38
Breech Mechanism Assembly 38
Loader Assembly 38
Recoil Cylinder Assembly 38
Part Two-Operation of Gun Mechanism 41
Firing the First Round 41
Loading 41
Ramming 42
Firing the Round 43
Firing Automatically 44
Action of Breech Mechanism and Loader During Recoil 44
Action of Breech Mechanism and Loader During Counterrecoil 46
Action of the Recoil System 46
CHAPTER III
BARREL ASSEMBLY 51
40MM Gun Barrel, Mark 1 51
Water Jacket 52
Flash Hider 52
Recoil Spring 52
CHAPTER IV
SIGHTS 53
Description of Sights 53
40MM Sight, Mark 3 53
40MM Sight, Mark 4 53
Pointer's and Trainer's Sights 53
Use of Sights 54
Alignment of Sights 55
Checking the Alignment 55
Adjustments 56
 

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CHAPTER V
CYCLIC OPERATION OF THE GUN MECHANISM 57
Feeding the Round 58
Cocking the Rammer 58
Ramming the Round 61
Closing the Breech 61
Firing the Round 62
Ejecting the Case 62
CHAPTER VI
LOADING AND UNLOADING 65
Instructions for Loading 65
Instructions for Unloading 65
CHAPTER VII
CASUALTIES 69
Casualties Which Can Be Corrected Quickly and Easily 69
Clip Placed in Loader Improperly 69
Firing Pedal Not Fully Depressed 69
Other Casualties 69
Live Round Drops Out Into Case Chute 69
Live Round In the Gun or On the Rammer Tray 70
Live Round Still Above Star Wheels 71
Star Wheels Locked 71
Long Recoil 72
Twisted Crankshaft 72
CHAPTER VIII
FUNCTIONAL CHECK-OFF LIST 77
Daily Check 77
 

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CHAPTER IX
ASSEMBLY AND DISASSEMBLY 78
Barrel Assembly 80
Removal of the Barrel Assembly 80
Installation of the Barrel Assembly 81
Disassembly of the Barrel Assembly 81
Assembly of the Stripped Barrel Assembly 84
Recoil Cylinder Assembly 87
Removal of the Recoil Cylinder Assembly 87
Installation of the Recoil Cylinder Assembly 87
Disassembly of the Recoil Cylinder Assembly 87
Assembly of the Stripped Recoil Cylinder Assembly 89
Loader Assembly 92
Removal of the Loader Assembly 92
Installation of the Loader Assembly 94
Removal of the Rammer Tray 94
Installation of the Rammer Tray 95
Disassembly of the Rammer Tray 95
Assembly of the Stripped Rammer Tray 96
Disassembly of the Loader Assembly 97
Assembly of the Stripped Loader Assembly 104
Housing Assembly 110
Removal and Disassembly of the Housing Assembly 110
Assembly and Installation of the Housing Assembly 113
Breech Block Assembly 115
Removal and Disassembly of the Breech Block Assembly 115
Assembly and Installation of the Breech Block Assembly 118
Slide Assembly 120
Removal and Disassembly of the Top Door 120
Assembly and Installation of the Top Door 121
Removal and Disassembly of the Side Door 122
Assembly and Installation of the Side Door 123
Removal and Disassembly of the Bottom Cover 123
Assembly and Installation of the Bottom Cover 124
Removal and Disassembly of the Rear Door 124
Assembly and Installation of the Rear Door 124
 

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ASSEMBLY AND DISASSEMBLY (Continued)
Removal and Disassembly of the Hand Operating Mechanism 126
Assembly and Installation of the Hand Operating Mechanism 127
Removal and Disassembly of the Trigger Mechanism 128
Assembly and Installation of the Trigger Mechanism 128
Removal of the Extractor Release Lever 130
Installation of the Extractor Release Lever 130
Removal of the Buffer Pad 130
Installation of the Buffer Pad 130
Removal of the Breech Block Locking Bolt Bracket and the Housing Stop 130
Installation of the Housing Stop and the Breech Block Locking Bolt Bracket 130
Sight Assembly 131
Removal of Sight Assembly 131
Installation of Sight Assembly 131
Disassembly of the Sight Assembly 131
Assembly of the Stripped Sight Assembly 132
CHAPTER X
PARTS LIST 133
Barrel Assembly 134
Recoil Cylinder Assembly 135
Loader Assembly 137
Housing Assembly 145
Slide Assembly 148
Sight Assembly 156
CHAPTER XI
APPENDIX 158
Data Sheet 158
Lubrication Instructions 159
Filling the Recoil Cylinder 160
Variations 161
 

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LIST OF ILLUSTRATIONS

1. 40MM Antiaircraft Guns 14
2. Slide Assembly 17
3. Trunnion Diagram 18
4. Trigger Mechanism and Hand Operating Mechanism 19
5. Firing Selector Lever Positions 20
6. Top Door 21
7. Barrel Lock 21
8. Side Door Cam Action 22
9. Rear Door 23
10. Housing-Broken Open View 24
11. Housing-Front View 25
12. Housing-Side View 26
13. Housing-Disassembled 27
14. Safety Plunger 28
15. Breech Block-Disassembled 29
16. Loader Assembly 30
17. Feed Pawls and Stop Pawls 31
18. Loader Assembly-Rear View 32
19. Rammer Tray 33
20. Loader Mechanism 35
21. Sectional View of Recoil Cylinder 37
22. Gun Mechanism Assembly 39
23. Cocking Mechanism 42
24. Breech Closed 43
25. Breech Block-Firing 44
26. Breech Block-Cocking 45
27. Operation of the Recoil Cylinder Assembly 49
28. Barrel Assembly 51
29. Breech End of Barrel 52
30. Sight-Mark 3 53
31. Sight-Mark 4 54
 

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32. Mark 3 Sight Alignment Diagram 55
33. Mark 4 Sight Alignment Diagram 56
34. Feeding the Round 59
35. Cocking the Rammer 59
36. Ramming the Round 60
37. Closing the Breech 60
38. Firing the Round 63
39. Ejecting the Case 63
40. Using the Round Releasing Tool 66
41. Using the Pusher Tool 67
42. Longitudinal Section of Gun 73
43. Lateral Sections Through Loader-I 74
44. Lateral Sections Through Loader-II 75
45. Lateral Sections Through Housing 76
46. Special Tools Used in Assembly and Disassembly 78
47. Recoil Spring Compressor Assembly 82
48. Recoil Spring Compressor Disassembled 83
49. Removing the Recoil Cylinder 86
50. Removing the Needle Valve 88
51. Recoil Cylinder Disassembled 90
52. Tightening the Throttling Bushing 91
53. Rear Door-Open 92
54. Removing the Tray Bolt 92
55. Using Loader Lifters 93
56. Removing the Rammer Tray 94
57. Tray Pawls 94
58. Removing Bearing Screw and Rammer Shoe Nut 95
59. Disassembly of the Rammer Shoe 95
60. Feed Control Mechanism 97
61. Removing a Star Wheel Plunger 98
62. Removing a Star Wheel 99
 

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63. Removing the Feed Pawl Holder 100
64. Removing the Catch Head 101
65. Catch Head 101
66. Removing the Outboard Rammer Cocking Lever 102
67. Removing the Catch Levers and the Bearing Pin 103
68. Stop Pawls 106
69. Feed Pawls 107
70. Removing the Extractors 110
71. Removing the Tray Bolt Spring Seat 112
72. Removing the Barrel Stop 112
73. Disassembly of the Closing Spring 113
74. Opening the Side Door 115
75. Using the Hand Extractor Tool 116
76. Removing the Breech Block 117
77. Removing the Firing Spring Cover 118
78. Removing the Outer Cocking Lever 118
79. Top Door Assembly 120
80. Installing the Top Door 121
81. Removing the Side Door Catch 122
82. Bottom Cover 123
83. Removing the Recoil Indicator 125
84. Removing the Case Deflector Brackets 125
85. Interior of the Slides 126
86. Detail of the Interior of the Slide 129
 

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40MM ANTIAIRCRAFT GUNS

Figure I
Figure 1

 

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Chapter I


INTRODUCTION

The 40MM Antiaircraft Gun, Figure 1, consists of the machine gun mechanism, the gun barrel, and the sights. The 40MM Machine Gun Mechanisms are assembled in pairs. The left mechanism is designated Mark 1 or Mark 1, Mod. 1, and the right mechanism is designated Mark 2 or Mark 2, Mod. 1. The only differences between the two Marks of mechanisms are those required by the twin assembly. The only difference between the Mod. 0 and Mod. 1 is that certain parts are not interchangeable between the two. 40MM Gun Barrels, Mark 1, are used with all mechanisms. The 40MM Sight, Mark 3, is used on the 40MM Twin Mount; the 40MM Sight, Mark 4, is used on the 40MM Quadruple Mount.

The design of these mechanisms and barrels is essentially that of the Swedish Bofors 40MM Antiaircraft Gun. The design provides for a rapid fire, recoil operated, automatic mechanism, with a maximum cyclic rate of approximately 160 rounds per minute.

 

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Chapter II


DESCRIPTION MECHANISM OPERATION

Part One-Description of Gun Mechanism

The 40MM Machine Gun Mechanism consists of five principal components. They are the Slide Assembly, Breech Mechanism Assembly, Loader Assembly, Recoil Cylinder Assembly, and Barrel Assembly (excluding barrel).

A. SLIDE ASSEMBLY

The slide, Figure 2, is a steel casing which positions and provides working surfaces for other parts of the mechanism. It has a trunnion, trigger

Figure 2
Figure 2

mechanism, hand operating mechanism, top, side, and rear doors, bottom cover and extractor release lever. A Mark 1 and a Mark 2 Mechanism are bolted together and are supported at the trunnions, one on the left side of the Mark 1 Mechanism, and one on the right side of the Mark 2 Mechanism. An elevating arc is attached to the bottom surfaces of the slides.

 

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TRUNNION DIAGRAM

Figure 3
Figure 3
The trunnion on the left is part of the Mark I Mechanism, the one on the right is part of the Mark 2 Mechanism.

1. Trunnion

The trunnion, Figure 3, contains the firing plunger and crank of the trigger mechanism. The trunnion of the Mark 1 Mechanism is provided with a gear sector to transmit gun elevation to the firing cut-out mechanism of the mount.

2. Trigger Mechanism

The trigger mechanism, Figure 4, is on the left side of the Mark 1 Mechanism on the right side of the Mark 2 Mechanism. It is operated by the firing plunger in the trunnion. The trigger is a vertical arm, pivoted near its center, and is located inside and at the rear of the slide, where it contacts and controls the movement of the rammer control spindle arm of the loader. The trigger is operated by a pawl on the firing lever. Motion of the trigger and firing mechanism is limited by the cam on the firing selector lever shaft, as shown in Figure 5. When the firing selector lever is at SAFE, motion of the trigger mechanism is prevented by the firing selector cam. When the selector lever is at AUTO FIRE, the trigger can be maintained in the firing

 

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position by the trigger mechanism. When the selector lever is at SINGLE FIRE, motion of the trigger mechanism can be sufficient to permit the pawl to trip and thus release the trigger. The trigger returns to its non-firing position after releasing the trigger catch lever in the loader. Thus in SINGLE FIRE the firing mechanism must be restored to its non-firing position before the trigger can again be operated.

Figure 4
Figure 4
Phantom positions of the rammer control spindle arm show how it contacts the trigger when the loader is placed in the slide. The position of the hand operating lever shaft is shown when the hand operating lever is in the rear catch bracket.

3. Hand Operating Mechanism

The hand operating mechanism, located on the same side of the slide as the trigger mechanism, provides the means for performing manually the operations required to prepare the gun for firing, operations which are otherwise accomplished automatically during recoil and counterrecoil. It

 

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FIRING SELECTOR LEVER POSITIONS

Figure 5.  Lever set on Safe, Auto Fire and Single Fire.
Figure 5

 

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consists of the external hand operating lever and associated internal linkage. The arm on the hand operating lever shaft, Figure 4, operates the cocking mechanism of the loader. A projecting lever of the shaft is pinned to the hand operating rod, which engages the toe of the breech mechanism outer crank to cock the breech mechanism. The lever also provides an interlock with the trigger mechanism to prevent firing when the hand operating lever is in the rear catch bracket.

4. Top Door

The top door, Figure 6, when open, disengages the barrel lock, Figure 7, and rotates the safety catch arm of the housing into position to engage a stop on the slide. This door must be left latched open when the barrel assembly is not in place, to prevent possible rearward movement of the housing.

However, the top door must always be latched closed before firing, in order to disengage the safety catch arm from the stop, thus preventing damage to the mechanism.

Figure 6
Figure 6
The top door, when open, disengages the barrel lock.

Figure 7
Figure 7
The barrel lock is shown in both open and closed positions.

 

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SIDE DOOR CAM ACTION

Figure 8
Figure 8
This transparent view shows the action of the cam surface of the side door on the outer crank. The crank is shown at the start and finish of its travel during recoil.

5. Side Door

The side door has a cam surface shown in Figure 8, to provide for the rotation of the outer crank of the breech mechanism during recoil.

This door must be opened to remove the breech block, closing spring, and cranks from the housing. However, the side door must always be locked closed before firing, so that the breech block will be lowered during recoil. Otherwise the breech block will be rammed into the front of the loader causing serious damage.

6. Rear Door

The rear door, Figure 9, has an opening to permit the recoil of the rammer tray and ejection of empty cases, and to facilitate removal of live rounds from the tray. It opens to permit removal of the loader and housing assemblies.

 

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REAR DOOR

Figure 9
Figure 9

To this door are attached case deflector brackets on which are mounted the case deflector, counterweights, and recoil indicator. This indicator measures the length of recoil, being contacted by the rear of the tray in recoil. It must be reset by hand.

7. Bottom Cover

The bottom cover provides access to the breech mechanism and permits removal of the breech block and associated parts.

8. Extractor Release Lever

The extractor release lever, located in the bottom of the slide, provides for manually releasing the extractors from the breech block.

 

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B. BREECH MECHANISM ASSEMBLY

The breech mechanism assembly consists of a housing assembly, breech block assembly and associated operating parts, and is a recoiling part of the gun mechanism.

1. Housing Assembly

The housing, Figures 10, 11, and 12, is a rectangular steel block with projecting bronze-covered bearing strips which ride in corresponding guides in the slide. The housing is provided with an interrupted thread for attaching the breech end of the barrel. The barrel lock is pivoted in the top of the housing

Figure 10
Figure 10
Broken open housing shows the relationship of the parts of this assembly.

 

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HOUSING-FRONT VIEW

Figure II
Figure II
This view of the housing shows the breech block in the closed position.

and locks the barrel in the assembled position when the top door is closed. The safety catch arm prevents rearward movement of the housing, when the top door is latched open, by contact against the stop fastened to the slide. A vertical slot is provided for the movement of the breech block. A transverse splined crankshaft keys together the outer crank, two inner

 

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HOUSING-SIDE VIEW

Figure 12
Figure 12 This is the complete assembly for Mark I Mechanism, as it appears from the rear outboard side.

cranks, and the cover of the breech block closing spring. These are shown in Figure 13. Cams on the inner cranks operate in slots in the sides of the breech block to open and to close and lock the block in the closed position. Additional cams on the inner cranks operate the outer cocking lever and the sear of the breech block. Extractors, operated by the downward stroke of the

 

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HOUSING DISASSEMBLED

Figure 13
Figure 13

breech block, extract the empty case from the barrel chamber, and lock the block in the open position against the action of the closing spring. The locking is accomplished by the hooks on the extractors engaging abutments on the breech block. The extractors are tripped by a live round catapulted into the chamber, or by manual operation of the extractor release lever. A spring loaded safety plunger, Figure 14, moves longitudinally in the housing and contacts a cam on the breech end of the barrel. This plunger moves forward

 

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SAFETY PLUNGER

Figure 14
Figure 14
The safety plunger is shown held back by the cam surface on the breech end of the barrel.

when the barrel is removed, enters a notch in the breech block, and locks the block in the open position. The plunger may be retracted to release the block by the use of a special tool inserted in the top of the housing.

2. Breech Block Assembly

The breech block, Figure 15, contains the firing pin and spring, inner and outer cocking levers, and the sear and sear spring. The inner cocking lever holds the firing pin in the cocked position or releases it. The outer cocking lever, which is splined to the inner cocking lever, is acted upon by the cam of the left inner crank to cock the firing pin. In the cocked position, the sear is forced outward by its spring, locking the inner cocking lever. In the firing position, the sear is forced inward by the right inner crank, releasing the inner cocking lever and the firing pin.

 

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BREECH BLOCK-DISASSEMBLED

Figure 15
Figure 15
The disassembled breech block shows all parts of this assembly and their identifying piece numbers. The firing hole bushing is not found in all blocks, as some have the firing hole drilled through the solid face of the block.

 

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C. LOADER ASSEMBLY

The loader assembly, Figure 16, consists of feeding, ramming and cocking mechanisms and their associated parts. It is located in the rear of the slide, resting on guides for easy removal, and is locked in place by the rear door.

Figure 16
Figure 16

 

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1. Feeding Mechanism

Live rounds are inserted in the loader in clips of four. The clips are automatically removed by the rear guide and ejected through the clip chute. The side frames include guides which contain the feed pawls and stop pawls, Figure 17, for automatically feeding single rounds onto the tray, during the counterrecoil stroke. The feed pawls are attached to a movable holder, which is positioned in the feed rod by a spring loaded plunger. In case of overload, as from a jammed round, the plunger will trip and release the feed

Figure 17
Figure 17

 

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pawl holder as the feed rod completes a stroke. The loader contains two star wheels which are pivoted at the forward ends and positioned at the rear ends by spring loaded plungers, Figure 18. The star wheels, Figure 20, are locked by catch mechanisms, which are tripped by pawls on the top face of the tray, or released by the hand operating mechanism. When a round is forced through the star wheels by the round above, the case spreads the star wheels sufficiently to pass through as it rotates the star wheels one-quarter turn.

Figure 18
Figure 18 Rear of loader; note transparent treatment of the crosspiece cover to show the star wheel plungers.

2. Rammer Tray

The loader contains the rammer tray, Figure 19, which is attached to the rear of the housing and recoils with it. The tray contains the rammer shoe, its levers, and the rammer spring. The rammer shoe, in the cocked position, is so located that the base of the live round, as it is forced through the star

 

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RAMMER TRAY

Figure 19
Figure 19 Below the rammer tray assembly, shown at the top of the picture, are the parts of a disassembled tray.

wheels, will fall into the slots of the rammer levers. The tray supports the live round before the round is catapulted, and provides a guide for the ejected case. Guides in the sides of the tray operate the feed rods during recoil and counterrecoil. The pawls on the top face of the tray unlock the star wheel catch mechanisms during counterrecoil at the instant the feed pawls force a live round through the star wheels onto the tray.

 

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3. Catch Levers

The rammer shoe is latched in the cocked position by one of three levers in the base of the loader. The three levers, Figure 20, are:

Center-Tray Catch Lever
Inboard-Loader Catch Lever
Outboard-Trigger Catch Lever

These levers are pivoted on a common shaft and are of varying length so that they release the rammer shoe in the following order: Tray catch lever, loader catch lever, trigger catch lever.

a. The tray catch lever is operated, through a rocker arm, by a cam on the bottom of the tray, and is in the releasing position whenever the tray is within about one inch of battery position.

b. The loader catch lever is maintained in the releasing position by a linkage in the rear guide, whenever the loader contains more than one round above the star wheels. Its purpose is to prevent the gun from firing the round on the tray, when there are fewer than two rounds above the star wheels to push another round onto the tray in counterrecoil. This lever is provided to prevent loss of automatic functioning of the mechanism, by latching the rammer shoe in the cocked position, if the loader is supplied with ammunition at a rate less than the rate of fire. This lever releases the rammer shoe automatically upon the insertion of additional clips of ammunition.

c. The trigger catch lever is controlled by the rammer control spindle, and the arm of the spindle is contacted and controlled by the trigger of the slide assembly. The trigger catch lever is placed in the releasing position whenever the trigger is in the firing position.

4. Cocking Mechanism

The loader contains the cocking mechanism which is operated by the hand operating mechanism of the slide. The cocking mechanism forces the rammer shoe to the rear and thus compresses the rammer spring. At the same time, the cocking mechanism rotates the catch mechanisms so that a round can be forced through the star wheels. Motion of the hand operating lever, beyond the position necessary to latch the rammer shoe, causes the rammer levers to move to the rear end of the slots in the tray. When the rammer levers are in this position, they are spread by the slots, so that a round can be inserted in or removed from the tray through the opening in the rear door.

 

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Figure 20, Page 35.
Transparent view of loader.
Important loader mechanism parts are shown in their proper relationship.
(Large image is on separate page.)
 

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D. RECOIL CYLINDER ASSEMBLY

The recoil cylinder assembly, Figure 21, provides the necessary retarding force during the recoil and counterrecoil strokes to limit the length of recoil and to control the velocity of counterrecoil. The recoil cylinder is attached under the forepart of the slide, and the piston rod is attached to lugs on the housing.

Figure 21
Figure 21 The recoil cylinder shown in the illustration has been cut open to reveal the relationship of its parts.

1. Piston Rod

The piston rod is attached to the housing and moves in recoil with it. The forward end of the rod is enlarged to form a piston, and contains a throttling bushing which provides an orifice for liquid passing between it .and the throttling rod. The inside surface of the piston rod has two grooves, tapered at each end, which control the flow of liquid around the valve seat.

2. Throttling Rod

The throttling rod is attached to the body of the recoil cylinder at the front end. It varies in diameter throughout its length, tapering to its smallest diameter near the front end, thus varying the opening between the rod and the bushing. The rear end of the rod carries the check valve which permits flow of recoil fluid through it only during the recoil stroke.

3. Needle Valve

The needle valve fits into a valve seat inside the throttling rod. By varying the setting of this valve, the velocity of counterrecoil, and consequently, the rate of fire of the gun can be controlled.

 

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4. Recoil Fluid

The fluid used in the recoil cylinder is specified by O.S. 1324. This mixture is approximately sixty percent glycerine and forty percent water, with a small percentage of corrosion inhibiting compound added.

E. BARREL ASSEMBLY

For description of barrel assembly see Chapter III.

F. DIFFERENCES BETWEEN MARK 1 AND MARK 2 MECHANISMS

Since the two mechanisms are bolted together so that they elevate and depress as a unit, certain differences between the Mark 1 and Mark 2 Mechanisms are necessary.

1. Slide Assembly

To permit the gun to be loaded, cocked and fired, each mechanism has its hand operating lever, firing selector lever, trigger mechanism, and firing plunger on the outboard side of the slide. To allow access to the breech mechanism assembly, the side doors are on the outboard side. The extractor release levers are also placed near the outboard side of each slide. The top doors differ in having their catches on the inboard side.

2. Breech Mechanism Assembly

The housings in the Mark 1 and Mark 2 Mechanisms differ in having their breech closing spring assemblies mounted on the inboard side, while the outer cranks, the extractor spindle arms, and safety catch arms are on the outboard side. The breech closing springs differ in the direction of their spirals; the closing spring in the Mark 1 Mechanism is wound so that it increases in diameter in a counter-clockwise direction, when viewed from the convex side of the spring; in the Mark 2 Mechanism, the spring is wound in the opposite direction.

Extractors and breech block assemblies are identical for the two Marks of mechanisms, and are, therefore, interchangeable.

3. Loader Assembly

The differences between the loaders in the Mark 1 and Mark 2 Mechanisms are those which are required to permit the cocking of the rammer and release of the catch mechanisms by the hand operating lever, the operation of the rammer control spindle arm by the trigger, and ejection of the ammunition clips through the outboard side of the slide.

4. Recoil Cylinder Assembly

The only difference between the recoil cylinders in the Mark 1 and Mark 2 Mechanisms is that the fill plug of each is positioned on the outboard side for easy access in filling the recoil cylinders.

 

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Figure 22, Page 39.
GUN MECHANISM ASSEMBLY
The transparent side shows how the other principal components of the mechanism fit together in the slide
(Large image is on separate page.)
 

40
 
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Part Two-Operation of Gun Mechanism

Before automatic operation of the gun mechanism can take place, the gun mechanism must first be cocked and a round fed onto the tray manually. When the trigger catch lever is depressed, the round is rammed into the chamber and fired. The resulting recoil and counterrecoil will now cock the gun mechanism and feed the rounds, thus providing automatic operation.

A. FIRING THE FIRST ROUND

1. Loading

To prepare the mechanism for firing, the hand operating lever is moved to the rear as far as possible and then forward to the rear catch bracket. This movement, Figure 23, performs three operations:

It rotates the rammer cocking levers which force the rammer shoe to the cocked position where it is held by the loader catch lever.

It operates the star wheel catch mechanisms, through a linkage and catch release pistons, to permit a quarter turn of the star wheels.

It lowers the breech block, through the hand operating rod, to a position which causes the extractors to lock the block down.

A clip of four rounds is now placed in the loader and pushed smartly home, rotating the star wheels and placing the lowermost round on the rammer tray with its base in the slots of the rammer levers. The clip is automatically detached from the rounds, and ejected through the clip chute of the loader.

These rounds press back the feed control lever, disengaging the loader catch lever from the rammer shoe. The trigger catch lever now holds the rammer shoe until released by the trigger mechanism. The hand operating lever is returned to the forward catch bracket, and the firing selector lever set on AUTO FIRE. The mechanism is now prepared to fire automatically as long as at least two rounds are in the loader above the star wheels.

 

42
 

COCKING MECHANISM

Figure 23
Figure 23 The illustration shows the position of the outer crank, the catch mechanisms, and the rammer cocking levers when the hand operating lever is moved fully to the rear.

2. Ramming

The inward motion of the firing plunger in the trunnion operates the trigger mechanism of the slide and disengages the trigger catch lever from the rammer shoe. The rammer shoe, with the rammer levers and the live round, is forced rapidly forward by the rammer spring. At the end of the rammer stroke, the rammer levers are spread by the slots in the tray, and the live round is released at high velocity and catapulted into the barrel chamber.

 

43
 

BREECH CLOSED

Figure 24
Figure 24 The extractors, caught by the round, have released the breech block, allowing it to be raised to the closed position.

3. Firing the Round

As the round enters the chamber, its base comes into contact with the extractors, pulls them forward, and disengages the extractor hooks from the breech block. The torsion of the breech block closing spring rotates the crankshaft with its attached cranks. The cams on the inner cranks raise the breech block to its closed position, as shown in Figure 24. After the breech block is raised, the inner cranks continue to rotate until the cams lock the breech block in its closed position. As the breech block is raised, the pressure of the cam of the left inner crank on the outer cocking lever is released. The inner cocking lever, and thus the firing pin, is now being held by the sear. During the last few degrees of motion of the inner cranks, the lower cam on the right inner crank forces the sear inward, releasing the inner cocking lever, and thus the firing pin. The firing spring then forces the firing pin into

 

44
 

BREECH BLOCK-FIRING

Figure 25
Figure 25
The breech block is shown in the closed position. Transparent treatment of the block shows the relationship of the parts of the assembly.

the primer. This is illustrated in Figure 25. The firing pin rotates the freely moving inner and outer cocking levers during its stroke.

B. FIRING AUTOMATICALLY

1. Action of Breech Mechanism and Loader During Recoil

During recoil, the cam on the side door acts on the outer crank to rotate the crankshaft and the inner cranks. The first movement of the inner cranks retracts the firing pin and unlocks the breech block, Figure 26. To do this, a

 

45
 

BREECH BLOCK-COCKING

Figure 26
Figure 26
The breech block is cocked during recoil, and the firing pin held back by the inner cocking lever.

cam on the left inner crank depresses the outer cocking lever, and thereby rotates the inner cocking lever, which retracts the firing pin. As the lower arm of the inner cocking lever clears the notch in the sear, the sear spring moves the sear to the right, so that it is held in position to lock the inner cocking lever. In the event of failure of the sear spring, motion of the sear to the right is insured by a cam on the left inner crank. Locking of the inner cocking lever will occur as the breech block is closing, when the outer cocking lever is released by the cam of the left inner crank.

Further rotation of the inner cranks lowers the block into contact with the toes of the extractors, rotating the extractors in the housing. The extractors violently eject the empty case through the housing and loader, between the spread rammer levers, through the rear door, and into the case deflector.

 

46
 
They also lock the breech block in the open position, as shown in Figure 14, page 28.

During the recoil movement of the rammer tray, the feed pawls of the loader are raised above the next rounds, which are being held in position by the stop pawls. This motion is produced by the roller on the lower end of the feed rod being moved by the guides on the sides of the tray.

2. Action of Breech Mechanism and Loader During Counterrecoil

During counterrecoil, the breech block closing spring acts to move the breech block to the closed position, but motion of the block is stopped as the breech block is latched open by the hooks on the extractors. Consequently the outer crank is carried clear of the side door cam.

As the tray moves forward during counterrecoil, the pawls on the top surface rotate the catch heads, so that the star wheels can be rotated by the round which is fed between the star wheels and then onto the tray. The rounds in the loader are forced downward by the feed pawls, which in turn are operated by the guides of the tray. As the tray is moving forward, the rammer shoe is latched to the rear by the tray catch lever, in such a position that the base of the next live round above the star wheels will be caught by the slots in the rammer levers as the round is dropped onto the tray. The tray continues forward, and when it is about one inch from battery position, the cam on the bottom of the tray trips the rocker arm, which in turn trips the tray catch lever, releasing the rammer.

The cycle, beginning with the ramming of the live round, will be repeated if the trigger is kept in the firing position, and if the loader contains sufficient rounds, so that the loader catch lever is in the releasing position. If the trigger is kept in the firing position, but the loader contains insufficient rounds for operation of the loader catch lever, firing will cease, but will begin again immediately upon the insertion of additional rounds into the loader.

3. Action of the Recoil System

The recoil system is comprised of the recoil spring and the recoil cylinder. The recoil spring provides in counterrecoil the force necessary to return the gun mechanism to the battery position, cock the rammer, and feed a new round. The recoil cylinder controls the length of recoil and the velocity of counterrecoil.

As the barrel, housing, and tray recoil, the housing carries with it the recoil cylinder piston rod. As the piston rod is drawn to the rear of the recoil

 

47
 
cylinder over the throttling rod, Figure 27, liquid is forced from the rear to the front of the piston through the eight holes in the piston and through the throttling bushing. Liquid pressure forces the check valve to the rear, thereby opening the ports in the valve seat. Through these ports liquid now passes to the rear of the valve. Liquid also passes to the rear of the valve around the valve seat and through the tapered grooves inside the piston rod; through the ports in the throttling rod, over the needle valve, and through the seat. These effective flow areas set up a fluid resistance which retards recoil.

Meanwhile, the recoil spring has been placed under greater compression. When recoil ceases, the recoil spring returns the recoiling parts to battery position, and the piston rod moves forward along with the housing.

During counterrecoil, the piston rod moves forward over the throttling rod, and the check valve, under spring action and liquid pressure, masks the ports in the valve seat. Liquid at the rear of the valve seat is forced to the front of the valve seat through the grooves in the piston rod and through the bore in the center of the valve seat. The tapered grooves gradually reduce the flow space as the barrel approaches the battery position until finally no liquid can pass over the valve seat. The flow of liquid from the rear to the front of the seat is now restricted to the bore in the seat, over the needle valve, and through the ports in the throttling rod. Liquid in front of the piston is forced through the throttling bushing and the holes in the piston.

Finally, the front end of the piston enters the recess in the head of the throttling rod. This action forces the liquid between the head and piston through the space between the throttling bushing and the throttling rod. The front taper of the rod reduces the flow space as the barrel approaches the battery position. This action and the tapered grooves inside the piston rod further reduce the speed of counterrecoil.

At the end of counterrecoil, the buffer pad provides a positive stop at battery position for the recoiling parts of the gun mechanism.

The length of recoil is measured by the recoil indicator, attached to the rear door. The indicator is operated by the rear surface of the rammer tray. A length of recoil of approximately 7.2 inches is required to insure automatic operation of the mechanism. Shorter recoil may be insufficient to permit the tray pawls to operate the catch mechanisms of the star wheels, thus preventing feeding. Short recoil may also cause a jam on the tray by causing the star wheels to start to rotate before the ejected case has had time to clear them. The maximum operating length of recoil is about 8.3 inches and is limited

 

48
 
by the design of the throttling rod and bushing in the recoil cylinder. Normal recoil should lie within the above mentioned limits, and will vary somewhat with the elevation of the gun, and with the quantity, specific gravity, and temperature of the recoil fluid.

The velocity in counterrecoil, and thus the rate of fire, is determined by the setting of the needle valve of the recoil cylinder. The setting of this valve has no appreciable effect on the length of recoil. The needle valve is adjustable at the forward end of the recoil cylinder.

OPERATION OF
THE RECOIL CYLINDER
ASSEMBLY

Figure 27

The top sectional view shows the recoil cylinder when the gun is at battery position, and the other two views show the flow of fluid during the recoil and counter-recoil strokes. The red areas represent fluid under high pressure, while the yellow areas represent fluid under low pressure.

Arrow pointing right.

 

49
 
Figure 27, Page 49.
OPERATION OF THE RECOIL CYLINDER ASSEMBLY
(Large image is on separate page.)
 

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