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20 MM Antiaircraft Gun, OP 911, 1943, is a Navy service manual for the anti-aircraft gun of choice during the early years of WW II.

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ORDNANCE PAMPHLET NO. 911
20 mm. A.A. GUN
20 MM. MACHINE GUN MECHANISMS MARKS 2 AND 4 20 MM. GUN BARRELS MARKS 2, 3, 4, AND 4 MOD. 1 20 MM. SIGHTS MARKS 2, 4, 4 MOD. 1, AND 5
20 MM. MAGAZINES MARKS 2 AND 4
20 MM. SHOULDER RESTS MARKS 2, 4, 5 AND 5 MOD. 1
Department of the Navy Bureau of Ordnance
DESCRIPTION
MARCH, 1943

 

NOTICE

Please make the following corrections on the pages listed below:

PAGE NO. CORRECTION
85 Tenth line up from bottom of page-change-(300021-6) to read (300021-7).
86 Figure 70 at top of page-change (300021-6) to read (300021-7).
87 Twelfth line down from top of page-change-(300021-6) to read (300021-7).
87 Last paragraph on page-Alternate Rear Sight Pin Tips-Mark 5 Sight-delete this entire paragraph.
 

ORDNANCE PAMPHLET NO. 911

20 mm. A.A. GUN


20 MM. MACHINE GUN MECHANISMS MARKS 2 AND 4
20 MM. GUN BARRELS MARKS 2, 3, 4, AND 4 MOD. 1
20 MM. SIGHTS MARKS 2, 4, 4 MOD. 1, AND 5
20 MM. MAGAZINES MARKS 2 AND 4
20 MM. SHOULDER RESTS MARKS 2, 4, 5 AND 5 MOD. 1

DESCRIPTION

Logo Department of the Navy, Bureau of Ordnance

MARCH, 1943

Return to Code M245F, File Copy No., Oct 11, 1966

 

 Photo of 20 mm. A. A. Gun on Mark 4 Mod. 3 Mount
20 mm. A. A. Gun on Mark 4 Mod. 3 Mount

 

3
 

PREFACE

This Ordnance Pamphlet, No. 911, is a revision of the Gun Section of Ordnance Pamphlet No. 813 and describes the construction, operation, and maintenance of the 20 mm. A.A. Guns.

Additional Ordnance publications relating to the use of the 20 mm. A.A. Guns are:

O.P. 909 -20 mm. Mounts Mark 2, Mark 2 Mod. 1, Mark 2 Mod. 2; Mark 4, Mark 4 Mod. 1, Mark 4 Mod. 2, Mark 4 Mod. 3; Mark 5, Mark 5 Mod. 1, Mark 5 Mod. 3, Mark 5 Mod. 4; Mark 6; Mark 10 and Mark 10 Mod. 1.
O.P. 945 -20 mm. A.A. Range Tables.
O.D. 4429 -Gun Sight Mark 14 Mod. 2-Installation and Operation.
SK. No. 103308 -20 mm. Machine Gun and Mount Index.
Ordalt No. 1186 -20 mm. Cradles Marks 2 and 4-Instructions for Necessary Modification for Assembly of Gun Sight Mark 14.
Ordalt No. 1226 -20 mm. Machine Gun Mechanism Marks 2 and 4
20 mm. Hand Grips and Shoulder Rests Marks 2 and 4-Instructions for Adding Drainage Holes.
Ordalt No. 1269 -20 mm. Cradle Mark 2 and Mark 2 Mod. 1.
20 mm. Cradle Mark 4 and Mark 4 Mod. 1-Instructions for Installation of Cocking Sheave.
Ordalt No. 1300 -20 mm. Shield Mark 2; 20 mm. Shield Mark 4-Instructions for Modification of Shield Brackets.
Ordalt No. 1366 -20 mm. Cradle Mark 2 and Mark 2 Mod. 1.
20 mm. Cradle Mark 4 and Mark 4 Mod. 1-Instructions for Cradle Modification to Facilitate Disassembly.
Ordalt No. 1395 -20 mm. Shields Marks 2 and 4-Improvement of Sight.
Ordalt No. 1398 -20 mm. Mounts Marks 2 and 4-Changes to Improve Lubrication.
Ordalt No. 1457 -Modification of 20 mm. Sight Mark 4 Mod. 1.

The Mark 1 Gun was the original type made in Switzerland. The Mark 2 is the improved version made in England and also being made in U. S. A. The Mark 4 Gun is almost identical with Mark 2 except for slightly different manufacturing limits. Both Marks 2 and 4 Guns will be found in service in the U. S. Navy, but eventually only the Mark 4 will be manufactured in this country.

 

4
 

INTERCHANGEABILITY

There is very little difference, from the operating point of view, between

Mark 2-British
Marks 2 and 4-U.S.N.

All of the above three Marks of assembled guns are interchangeable as units. There are certain differences in the Marks 2 and 4 guns that affect interchangeability of parts. When individual parts are not interchangeable between Marks 2 and 4, the assembly or the subassembly containing these parts is usually interchangeable.

AMMUNITION

The ammunition is common for Marks 1 and 2 (British) and Marks 2 and 4 (U. S. N.) Guns.

 

5
 

TABLE OF CONTENTS


NOTE-Plate 1 is the folded insert of the gun assembly at page 160.

Text
Page
Illus.
Fig. No.
Frontispiece 2
Preface 3
Chapter 1
DATA
Gun Firing Rate 11
Magazine Capacity 11
Gun Barrel Data 11
Range Data 11
Weights of Components 12
Chapter 2
GENERAL DESCRIPTION OF
GUN OPERATION
Features 15 1
Principle of Construction 15 2
Firing Features 15
Gun-Construction-General Description 15 Plate 1
Magazine-General Description 15 7
Mount 15
Operating Features 17 3-4-5-6
Chapter 3
GUN MECHANISM
Trigger Mechanism 23
Recoiling Mechanism 23
Gun Barrel Nomenclature 23 9
Gun Barrel Construction 25
Gun Barrel Replacement 25 10 and 12
Barrel Stop Pin 26 11
Barrel Locking Gear 26 8
Double Loading Stop 26 20 to 23
Breech Casing 27
Gun Mounting Attachments 28 13
Magazine Catch Gear and Ejector 29 14 to 18
Ejector and Cartridge Ejection 32 19
Magazine Interlock Gear Operation 33 16-17
Hammer Plate Operation 34 44-45
Double Loading Stop Operation 34 20 to 23
 

6
 
Chapter 4
TRIGGER MECHANISM
Text
Page
Illus.
Fig. No.
Trigger Mechanism 39 24
Trigger Function 40
Cocked Position 42 24-25
Firing Action 42 26 to 31
Safe/Fire Gear 47 32
Magazine Interlock Gear Function 48
Magazine Interlock Gear Action 48 33 to 42
Magazine Interlock Withdrawal When Unshipping Magazine 52
Chapter 5
RECOILING PARTS AND BUFFER
Components 57 43
Striker Gear 58 44-45
Breech Block 59 44-45
Breech Bars 59 43
Barrel Springs and Case 59 43
Recoil Buffer Springs 61 46
Trigger Buffer Springs 61 24
Chapter 6
MAGAZINE
Operation 65 47
Feed Action 65 47
Magazine Clock Spring 65 47
Cartridge Feeder 65 47
Spring Tension Limits 65 47
Loading 66 48 to 56
Completely Tensioning a Magazine Spring 70 58
Spring Tension Indicator 71 47
Slight Initial Tensioning of a Magazine Spring 71 58
Relieving the Magazine Spring Tension 72 59 to 61
Unloading a Fully Tensioned and Completely Filled Magazine 73 62
Unloading a Completely Filled Magazine Having Initial Spring Tensioning 73 63
Unloading a Fully Tensioned and Partly Filled Magazine 73 62
Stripping a Magazine 74 64 to 67
Reassembling a Stripped Magazine 77
Changing Magazines 78
Magazine Maintenance 78
Magazine Corroded or Dry 78
Magazine Clock Spring-Care of 78
Magazine Interlock Bolt-Care of 78 35
Magazine Lubrication 78
 

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Chapter 7
SIGHT
Text
Page
Illus.
Fig. No.
Description 82 68-69
Use of Sights Mark 2, Mark 4, and Mark 4 Mod. 1 83
Shipping and Unshipping the Sight-Mark 4 and Mark 4 Mod. 1 83
Preparing Sight for Stowing-Mark 4 and Mark 4 Mod. 1 83
Replacement of Foresight-Mark 2, Mark 4, and Mark 4 Mod. 1 85
Mark 5 Sight-Description 85 69
Mark 5 Sight-Operation 85
Mark 5 Sight-Servicing 87 70
Alternate Rear Sight Tips 87
Bore Sighting 88 71-72
Chapter 8
SHOULDER REST
Marks 2 and 4 Shoulder Rest Assembly. 96
Mark 5 and Mark 5 Mod. 1 Shoulder Rest Assemblies 96 73
Vertical Adjustment 97 73
Lateral Adjustment 97
Shoulder Pad Angular Adjustment 97
Instructions for Use 97 74 to 77
Adjustment of Clamping Levers 101
Chapter 9
AMMUNITION
Ammunition Types 104
Loaded and Fuzed Projectile with Tracer 104
Loaded and Fuzed Projectile without Tracer 104
Blind Loaded and Tracer Type Projectile 104
Blind Loaded and Plugged Projectile 104
Propellant 104
Ammunition Construction 105
High Explosive Shell 105
Ammunition Packing 105
Unpacking Cartridges. 105
Inspection of Cartridges 105
Greasing Ammunition 105
Dropping Cartridges 106
Cartridge Bag 106
Naval Ammunition Depot 106
Chapter 10
GUN MAINTENANCE INSTRUCTIONS
Cleaning and Oiling of Gun 108
When to Clean and Oil 108
Lubricating Oil to be Used 108
 

8
 
Chapter 10 (Contd.)
GUN MAINTENANCE INSTRUCTIONS
Text
Page
Illus.
Fig. No.
Double Loading Stop Parts Lubrication 108 78
Double Loading Stop Gear and Buffer Corrosion 108 78 and 80
Securing Bolt Lubrication 108
Barrel and Barrel Spring-Care of 109
Barrel Bore and Firing Chamber-Care of 109
Muzzle Covers 110
Stoppages-How to Clear 110
Use of Cartridge Extracting Tool 110 79
Life of Barrel 111
Gun Barrel Replacement 112
Gun Barrel Removal 112
Gun Barrel Installation 112
Violent Recoils 112
Buffer Corrosion 113 80
Gun Mounted on Submarine 114
Cleaning 114
Lubrication 114
List of Gun Casualties 115
Chapter 11
COCKING AND UNCOCKING THE GUN MECHANISM
Cocking Sheave Method-Mounts With Adjustable Trunnions 122 81-82
Cocking Sheave Method-Fixed Height Mounts 125
Cocking Rope Method-Without Cocking Sheave 125
Cocking Lanyard Mark 3 126
Hook and Rope Cocking Tool Mark 4 126
Block and Tackle Mark 2. 126 83
Ratchet Type Cocking Tool Mark 1 127 84
Chapter 12
PARTIAL STRIPPING
(See Plate 1, Page 160)
Gun Tools 130 85
Breech Face Piece Removal 131 86-87
Breech Face Piece Installation 134 88
Barrel Spring Case and Cotter Securing Bolt and Retainer Removal 135 89 to 92
Barrel Spring Removal 137
Hand Grip Removal 137
Trigger Partial Stripping 137 101
Breech Block, Bolt and Sear Removal 138 101
Breech Face Piece Removal 138 87
Striker Gear Removal 138 95
 

9
 
Chapter 12 (Contd.)
PARTIAL STRIPPING
Text
Page
Illus.
Fig. No.
Barrel Assembly Removal 138
Stripping the Gun Barrel Assembly 138
Barrel Locking and Double Loading Stop Removal 138
Buffer Removal 139 93
Ejector and Magazine Catch Removal 140 94
Inspection and Adjustment after Partial Stripping 142
Striker Pin Protrusion 142 95
Hammer and Hammer Plate Wear 143 95
REASSEMBLING GUN
Magazine Catch Lever and Ejector Reassembly 143 94
Barrel Locking Gear and Double Loading Stop Reassembly 143 96-Plate 1
Buffer Reassembly 144 93
Barrel Replacing 145 97-98
Striker Gear Reassembly 146 95
Breech Block, Bolt and Sear Reassembly 147 43 and 101
Trigger Hook Casing and Cover Plate Reassembly 147 101
Hand Grip Reassembly 147 Plate 1
Test of Operation before Further Reassembly 147
Breech Bar and Barrel Spring Reassembly (Rectangular Wire Type) 147 99
Breech Bar and Barrel Spring Reassembly (Round Wire Type) 147 100
Chapter 13
COMPLETE STRIPPING OF GUN
Stripping (Continuation of Partial) 151
Trigger Casing Gear Removal 152 101-102
Parallelogram 152 101-102
Parallelogram Spring Box 152 101-102
Trigger Gear in Breech Casing Removal 152 101-102
Safe/Fire Gear Stripping 154 32
Magazine Catch and Interlock Gear Stripping 154 94
Trigger Pawl Tripping Bolt Stripping 154 Plate 1
Magazine Interlock Rod Guide Block Removal 154 Plate 1
Breech Pawl Removal 154 Plate 1
Hammer Plate Stripping 154 Plate 1
Hand Grips Retaining Catch 154
Barrel Spring Seating Ring Retaining Catch Stripping 154 Plate 1
Breech Casing Barrel Stop Pin Stripping 155 Plate 1
Trigger Intermediate Lever Plunger Stripping 155 Plate 1
REASSEMBLING GUN
Trigger Intermediate Lever Plunger Reassembly 156 Plate 1
Barrel Stop Pin in Breech Casing Reassembly 156 Plate 1
Barrel Spring Seating Ring Retaining Catch Reassembly 156 Plate 1
Hand Grip Retaining Catch Reassembly 156 Plate 1
 

10
 
Chapter 13 (Contd.)
REASSEMBLING GUN
Text
Page
Illus.
Fig. No.
Hammer Plate Reassembly 156 Plate 1
Breech Pawls Reassembly 156 Plate 1
Magazine Interlock Rod Guide Block 156 Plate 1
Trigger Pawl Tripping Bolt 156
Magazine Interlock Fork 156
Safe/Fire Gear Reassembly 157 Plate 1
Trigger Pawl, Pawl Holder, Trigger Crank 157
Trigger Intermediate Lever 157
Trigger and Trigger Hook 157
Parallelogram 157
Trigger Hook Holder and Buffer Spring 157
PARTS LIST
Marks 2 and 4
Instructions for Ordering Replacement Parts 161
Gun Barrel 163
Gun Mechanism 164
Shoulder Rest 178
Sight 188
Magazine 197
Gun Tools and Accessories 204
Cocking Tools 208
Cross Index to OE and Bureau of Ordnance Numbers 215
Addendum Number 1 Add 1-1
Addendum Number 2 Add 2-1
 

11
 

DATA-MARK 2 AND MARK 4


Chapter 1

GUN  
Firing Rate 450 Rounds per Minute
MAGAZINE
Capacity 60 Rounds
GUN BARREL
Weight -Ribbed 37 Pounds
Weight -Solid 46 Pounds
Length 4 Feet, 9 11/64 Inches
Caliber ".7898 (Approx. 20 mm.)
Number of Grooves 9
Twist of Rifling Right Hand, 1 turn in 36 Cal.
Locked by Rotate 60 Degrees and rotate locking lever into slot in barrel
Barrel Spring Compression (breech block in extreme forward position)
Rectangular Wire 72 Pounds
Round Wire 130 Pounds
Barrel Spring Compression (metal to metal contact between barrel spring case and buffer)
Rectangular Wire 535 Pounds
Round Wire 374 Pounds
Barrel Spring Compression, Full Recoil (metal to metal contact between buffer and breech casing)
Rectangular Wire 576 Pounds
Round Wire 396 Pounds
Buffer Spring Compression, 12 Springs, Full Recoil (metal to metal contact between buffer and breech casing)
Rectangular Wire 2508 Pounds
Round Wire 2220 Pounds
Total Compression Barrel Spring and Buffer Springs, Full Recoil (metal to metal contact between buffer and breech casing)
Rectangular Wire (Barrel and Buffer Springs) 3084 Pounds
Round Wire (Barrel and Buffer Springs) 2616 Pounds
RANGE
Maximum at 45 Degrees Elevation Approx. 4800 Yds.
 

12
 
WEIGHTS  
Gun Barrel and Mechanism (Ribbed Barrel) 141 Pounds
Gun Barrel and Mechanism (Solid Barrel) 150 Pounds
Sight-Marks 2, 4, and 4 Mod. 1 13 Pounds
Sight Mark 5 7 Pounds
Shoulder Rest-Marks 2 and 4 20 Pounds
Shoulder Rest-Mark 5 28 Pounds
Shoulder Rest-Mark 5 Mod. 1 25 Pounds
Magazine-Marks 2 and 4-Loaded 63 Pounds
Magazine-Marks 2 and 4-Unloaded 31 Pounds
 

13
 

Notes

 

14
 

COMPONENT PARTS


Exterior view showing general arrangement
of the Mark 4, 20 mm. A. A. Gun
Figure 1-Exterior view showing general arrangement of the Mark 4, 20 mm. A. A. Gun

 

15
 

MAJOR COMPONENTS-MARK 4 AND MARK 2


Chapter 2
GENERAL DESCRIPTION OF
GUN OPERATION

FEATURES

The Marks 2 and 4 Gun Mechanisms, as stated in the "Preface" on page 3, operate the same. They are for close range, high angle fire as an antiaircraft defense. Each is singly mounted upon a pedestal and is capable of being trained through 360 degrees. The elevation limits on the guns mounted on Mark 2, Mark 4, and Mark 5 Mounts are from 5 degrees below horizontal to 87 degrees above horizontal except on Mark 4 Mod. 3 on which elevation is 90 degrees. Those mounted on Mark 6 and Mark 10 Mounts have elevation limits from 15 degrees below horizontal to 90 degrees above horizontal.

PRINCIPLE OF CONSTRUCTION

The major difference between this gun and others is that the force of the explosion is absorbed in checking and reversing the forward movement of a relatively heavy bolt, or breech block, that is never locked. In most guns the force of the explosion is taken by the locked breech block and by the recoil cylinders and mechanism. See Figure 2.

FIRING

The gun fires automatically as long as the trigger is pressed and there is ammunition in the magazine. When the last round of each magazine is loaded into the gun and fired, the trigger mechanism is returned automatically to the cocked position regardless of the position of the trigger. This feature prevents the breech block mass from counterrecoiling on an empty gun after the last round is fired and, because there is no explosion to reverse the breech block, a recocking by hand would be necessary before firing could be continued. A Safe/Fire lever is fitted close to the right hand grip. See Figure 32. The rate of fire is approximately 450 rounds per minute.

The magazine is easily and quickly changed. It is retained by lugs on its fore end and a magazine catch at the rear. See Figure 18.

GUN

The gun is considered to consist of the following three units. Sec Figure 1.
1. The gun barrel
2. The machine gun mechanism
3. The shoulder rest and hand grips
The sight and magazine are considered as supplementary units required to complete the assembly.

MAGAZINE

The ammunition is supplied from a magazine that holds 60 rounds. The magazine is detachably mounted on the gun. It is filled, also serviced while removed from gun.

MOUNT

The mount includes the cradle in which the gun is fixed, the carriage which carries the oscillating parts, and the stand which is bolted to the deck. For complete information on mounts, see Ordnance Pamphlet No. 909.

 

16
 

COMPONENTS


Figure 2-Recoiling Parts
Figure 2-Recoiling Parts

 

17
 

OPERATING FEATURES


The more important features of the operating principle of the Mark 2 and Mark 4, 20 mm. A.A. Guns are as follows:

(A). The breech casing and barrel do not recoil.

(B). The recoiling parts are the breech block and the parts that connect it to a powerful two-piece barrel spring. See Figure 2. The breech block recoils and counterrecoils with a purely reciprocating action.

(C). The barrel spring is the only force tending to keep the breech block closed. There is no positive lock.

Loading Mechanism in cocked position.
Figure 3-Loading Mechanism in cocked position. Breech block in position to pick up cartridge when trigger is released

(D). Preparatory to opening fire, the breech block must be pulled back, see Figure 3 and gun assembly Plate 1, until, the sear (OE-1317) is held by the trigger hook (OE-1216). This compresses the barrel springs (OE-1320 and 1321) and causes a pull on the recoiling parts being held in the cocked position by the trigger hook.

(E). Pressing the trigger releases the breech block allowing it to move forward, under the pull of the barrel spring.

(F). A round of ammunition is picked up from the magazine by the breech block on its way forward and is carried toward the gun barrel. At a point about half way forward, the next round in the magazine forces the cartridge down into the lip of the breech face piece as shown in Figure 4.

 

18
 

Operation of Loading Mechanism about half way of forward stroke
Figure 4-Operation of Loading Mechanism about half way of forward stroke

(G). Just before the breech block reaches the fully forward position, a striker pin in the breech block is operated by a hammer in the breech block and fires the round. A hammer plate in the breech casing operates the hammer as the breech block travels on recoil and counterrecoil inside the breech casing. Figure 5.

(H). When the round fires, the gas pressure first absorbs the forward momentum of the breech block and then blows the latter backward, thereby compressing the barrel spring which absorbs the rearward momentum imparted to the breech block by the gas pressure. The breech block is blown backward until at full recoil, it is to the rear of the position at which the trigger hook catches it. See Figure 31.

(I). As each round fires, the empty cartridge case together with the breech block is blown back from the chamber by the gas pressure. Subsequently the empty case is tipped out of the breech face piece by striking against an ejector in the breech casing. See Figure 6. The next round in the magazine is forcing the empty cartridge downward.

Figure 5-Cartridge Fired
Figure 5-Cartridge Fired

 

19
 

Breech Block returning to cocked position Empty cartridge being ejected
Figure 6-Breech Block returning to cocked position Empty cartridge being ejected

(J). When all the rearward momentum has been absorbed by the barrel spring, the counterrecoil of the barrel spring forces the breech block forward again. On its way, the breech block picks up the next round from the magazine and the firing sequence is continued as long as the trigger is kept pressed or until the magazine is emptied.

(K). Whenever pressure on the trigger is released, the breech block is caught on the next attempt to commence its counterrecoil and is held in the cocked position. See Figure 3. This happens in all cases when there is ammunition in the magazine. In case the pressure on the trigger is maintained and the magazine becomes empty then the trigger mechanism is returned automatically to the cocked position regardless of the position of the trigger. See Figure 24.

(L). The magazine, see Figure 7, consists of a cylindrical drum containing a clock spring that forces a cartridge feed block around a spiral in the magazine forcing the rounds down into the magazine mouth, where they are picked up by the breech block as it counterrecoils. A detailed description of the magazine action is given on Page 65.

 

20
 

Magazine
Figure 7-Magazine

 

21
 

Notes

 

22
 

Notes

 

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