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 M09 DESCRIPTION MARCH, 1943 SHIELD CLAMPING LEVER-- --- COCKING WIRE ROPE ZO mm. A. A. Gun on Mark 4 Mod. 3 Mount PREFACE Thi:: OrC::::~.:-:cc Pa;..i=::~c: ;\o 911. is:" re\'ision of the Gun Section o( Ordnance Prk 6; Mark 10 and :Vlork 10 Mod. !. -20 mm. A.A. Range Tables. 0.0. H29 -Gun Sight Mark B Mod. 2-1n stallMion and Opemrion. st..:. ~o. 103308-20 mm. Ylachine Gun and Mount lndex. Ordalt No. 118&-20 mm. Cradles M.:t.l'ks 2 and 4-[nstn>Ctions ior :Neces:sary ModifiC:.1tion for .'\!;sembly of Cun Sight Mark B. Ordalt No. l226- 20 mm. Machine Gun Mechanism :".l:'l.rks 2 a:ld 4-20 mm. Hand Grip$ and Shuutd~: R. .. -sts :\larks 2 ~utd 4-ln:ilructions {or :\ddin~ Ur..inagc Hole~. Ordalt :-lo. 1260-20 mon. Cradle ~lark 2 and ~lork 2 :11od. I. 20 mm. Cradle )r::t.rk -4 o.nd ).lark 4: .:Vtod. 1-lnstructions for l11~t."l!lation of Cock1ng Shelve. Ord.n.lt Xo. 1300-20 mm. Shield ;\(ark 2:20 mm. Shidd 2\f:l.rk 4.-l;lscruccioll:s for ~£<>dific:uion ol Shic.C Br.~.<:keu. Ordalt Xo. 1366-20 mno. Cradle ~l~rk 2 >nd ~lark 2 )1od. I. 20 mm. Cr.~.dlc ).·lark 1: J.r.d ).J:Jrk -1. ).1<...-.d. 1- lnstructions for Cradle ;.\Iodili.c:don t'-' Facilic:ue Diuss~mbly. Ordalt No. 1395-20 mm. Shields Marks 2 and 4-lmprovement of Sigh c. Ordalt No. 1398-20 mm. Mounts :\larl.."S 2·and 4-Chal\ge$ co Improve Lubrication. Ordalt 2\'o. 14$i-2\lodi:!i.C;ttioa of 20 mm. Sight ).l;..rk 4 Mod. 1. The :\1ark l Gun was the origln3l type made in Swic~rland. The :vi'ark 2 is i:he improv'!d .. ~~~· n1adc in England and also being made io U.S. A. The Mark 4 Gun is almost identical w:;h 2\Ld.: 1 .:. . ~-• ior !lightly different mnnufacturing limits. Both :\larks 2 and 4 Guns will bt: found in s~rv:ke .n ~'1~: :._· ! :\o.vy, but eventuaJir only the :\1ark 4 wiH bt: manufactured in this cuuncry. \ L - INTERCHANGEABILITY There is very little difference, [rom the opero~ing point of view, between Mark 2- British Marks 2 and 4-U. S. N. All o[ the above three Marks or assembled guns nre inte:changeable as units. There arc certain differences in the Marks 2 and 4 guns that affect intcrchangeabiitt;- of parts. When individual parts are not interchangeable between Marks 2 and 4, the auembly or the !COn Barrel lhta . . . . . . . . . . • . . . . • . . • . . . .. . . . . . . . . . . . . . . . . . . . . . . . . 11 R~nge Data. . . . . . .. . . . . .. . . • . . . . . . . . . . . . . . . .. . . . .. . . . . . . .. . • .. . .. . . . . . . .. . 11 \\'eights of Components .........•....•.. .... ...... ............................. 12 Chapttr 2 CENER.-\L DESCRIPTION OF CU~ OPERATION Featuru...................................... . ................... . ·~ Principle of Construction . . . . . . . . . . . . . . . . . . . . • • . . . . . . . . . . . . . . . . . . . . . . • . . . 15 2 Firing Fe::.tures. . . . . . . . . . . . ......... , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Gun- Com;truction-Gencral Description. . . . . . . . . . . . . . ...................... . I• o. P!at< 1 Nlagazi.ne-General Descriplion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 ~1ount........... . . .........................•........ 15 Operating Fearu~es .................................................. . 17 3+5-o Chapter 3 Gl-:\ :\IECH:\.'\IS:\1 Trigge~ :\locoar.i;r:: 23 Recoiiir.g ~lec:-t.1:::sm 23 G"n Bar:el :"or::e"c:~:-~= 23 9 G. . m Barre: Co:1s:.:-.:c:~:-: :?~ Gun Barre: Re;:.k:-t=-::t:.: 25 !0 a:-:ci t? Ba~rel Stop Pin 20 11 Barrel Locking ~,, 26 8 s,c;: Double Loading 26 20 tO 23 Breech Caaing. . 27 Gu:1 Mounting .J..~:ac."':::-:~~!! 2S 13 ,"d ~laguiae Catch ~a: E'•=:c: 29 14 to 18 .,_ Ejector and Cartridge Ejec:i~n '? 19 Magazine Interlock ~ar Oooro::or. . 33 16-li Hamme: Plate Operation · 34 44-45 Double Loo.ding Stop Operatioo. 34 20 to 23 CONTENTS PAGe 5 TABLE OF CONTENTS Ch~pter 4 TRIGGER ~IECH.<\.:'IISM Jllus. fi&. ~o. Trigger Mechanism ....................... . 39 24 Trigger Function ................ . 40 Cocked Position .....•............. . . . . . . . . . . . . • . . . . • . . 42 24-25 Firing Action ...................... . 12 26 to 31 Snle/Fire Gear. .............. . .. ···•···· ............. H 32 Mn gazine Interlock Gear F :.mc:io~: 48 Magazine Interlock Gea: .>.c:ion 48 33 to 42 .~ I nga.z.m e I nter Io ck · \\""J :..r .c. .:a.,,·a... \\..~ e:: .l. ::.:.~ :.; ;:::.: -.; .'• I a;.:.:;:-.: .\•0 . .................. ' .. 52 RECOIU;\G P.\RTS .\.'\D Bt:FFER ("nmronentF ~7 43 .;s :;triker Gear. H-45 .. Breech Dlock . .......... . 39 44--15 B rccch Burs . . ........... . .;g 43 Barrel Springs and Case .. ~9 43 Recoil Buffer Springs .. 61 46 Trigger Buffer Springs .. ......... 61 2-! Ch3pter 6 Opcrntion. . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . .........•...... 65 47 Feed Action. . . . . . . . . . ...... , . . • . • . . • . • . . . . . . . . . . . . . . 65 47 Mng:uine Clock Spring. . . . . .. . . .. . .. . . . . .. . . . . . . . . . . . • . . . . 65 47 Cartridge Feeder. . . . . . . . . . . . . • . . • . . . . • . . . . . . . . . . . . . . . . 65 47 Spring Tension limits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 47 Loading.............. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . 66 48 to 56 Completely Tensioning a ~lagazine Spring . . . . . . . . . • . . . . iO 5S Spring Tension Indicator.... . . . . . . . . . . . . . . . ...... , ........ , il 47 Slight Initial Tensioning ol a :'>!ngazine Spring. . .......•..•..... i l 58 Relieving the Magazine Spring Tension. . . . . . . . . . ................ 72 59 to 61 Unloading a Fully Tensioned nnd Completely Filled :l!~gnzi~e ................... 73 62 Unlooding a Completely Filled Magazine Having lnitinl S~r:ng Tensioning ..... , .... 73 63 Unloading a Fully Tensioned and Pnrtly Filled Magazine..... . .................. 73 62 Stripping a Magazine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............... , .... 74 64 to 67 Re>ssembling a Stripped Magazine . . . . . . . . . . . . . . . . . . . , .................... 77 Chansing- Ylagazines . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . . . . . . . . . . . . . . . . . . . 78 )lagazine Maintenance. . . . . . . . . . . . . . . . .. . . . . . . . . . . , . • . . . . . . . . . . . . . . . 78 Magazine Corroded or Dry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . •................ iS Magazine Clock Spring-<:are or. ............. , ..... , , .......................... 78 ~!agazine Interlock Bolt-Care of. .............................................. 78 35 Magazine Lubrication ....................... ................................... 78 PAGE 6 CONTENTS TABLE OF CONTENTS Cb:~pter 7 Tut SICH.T Page Description......... .............................................. 82 68-69 Usc of Sights Mark 2, Mark 4, and ;>.lark 4 ;>.lod. I................... .. ...... 83 Shipping and Unshipping :he Sight- :-.lar;, 4 and ~lark 4 Mod. 1 ................... 83 Preparing Sight for Stowir.g-~lark 4 a~d ~lark ~ :\lod. I .......................... 83 Replacement of For0$ight-Mark ~- ~lark 4. and ~lark ~ Mod. 1 .... ..•............ 85 Mark.; Sight- Description......... . ............................ 55 69 Mark 5 Sight-Operation....... ........ . .............................. 85 ~lark 5 Si~rht-Servicing.. .. .. . .. . .. . .. . . . . . .. . .. .. .. .. . .. .. . . 87 70 Alternate Rear S!ght Tips...... . . . . . . . . . . . . . . . .. . . .. . . . .. . . . .. . $7 Bore Sighting......................... . .................... 88 ;1.12 Chapter 8 SHOt:lDER REST Marks 2 and 4 Shoulder Rest Assemb>: . . . . . . . . . . . . . • . . . . . . • . . . . . 96 )t' . . . . ...... . . ..................... 10<1 [A,dcd and Fuzed Projccile with T:-:1cer. .. .. .. .. • .. .. 1().1 Loaded and Fuzed Projec:.le without Tracer T;•pe Projectile ...... ... . 10<1 Blind Loaded ond Plu~ged Projectile .. . ............... 10-1 Propellant.. . ...... . 104 Ammunition Construction ............. . . •.••.•........ 105 High Explosive Shell. ................................ .. . ............... 105 Ammunition P:lcking. . . . . . . . . . . . . . . . . . . . . ...•.....•..... 105 L'np<~cking Cartridges .............................. . . . . . . . .105 Inspection or Cartridges.. .. .. .. . .. . .. . .. .. ............... .... 105 Grelting Ammun~tion.... . . . . . . . . . . . . . . . . . . . . . . . ........ ....... 105 ::>ropping Cartrid~rcs.. . . . .. . . . . . . . . . . . . . . . . . . . . . .. ..... . 106 -:··~die R~it ............................... .. . . . . . . . . .106 =".:.·•~ Am::~unition Depot ................. . .. ................ 106 Chapter 10 GUN ~1.-\L'\TEN.\XCE lNSTRt,;CTIOXS ::~:.:::; :."": -~ .:-:i ~! G..lr '\":·.!;- :: .... ~ ... - :."':: : .. --=-=· "'i CONTENTS TABLE OF CONTENTS Chapter 10 (Contd.) iext Jllus. GUN MAJNTEI"ANCE INSTRCCTIONS P::age: Fi&. No. Double Loading Stop Parts Lubrication. . . . . . . . . . . . . ................... . .108 78 Double Loading Stop Geo.r and SuiTer Corrosion . . . . . . . . . . . . . . . . ..... •.... .108 78 and SO Secur:ng Bolt Lub:ica

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