Chapter XI



40MM Machine Gun Mechanism  
Type of mechanism recoil-operated
Type of breech block vertical sliding wedge
Brake load 4,800 pounds
Length of recoil at 0° elevation approx. 7.5 to 8.0 inches
Rate of fire 130 to 150 rounds per minute
Gun Barrel  
Bore diameter in inches 1.573 inches
Length in inches (overall) 88.583 inches
Length in calibers 56
Depth of grooves .0225 inches
Twist of rifling increasing from one turn in 45 calibers at origin to one turn in 30 calibers at muzzle.
Interior Ballistics  
Chamber capacity 28.3 cu. inches
Travel of projectile 78.28 inches
Muzzle velocity 2,890 ft. per second
Service pressure (maximum) 19.5 tons per sq. inch
Recoil Cylinder  
Type Hydraulic
Kind of liquid (O.S. 1324) 60% glycerin, 40% water
Type Fixed
Weight of one complete round (brass case) 4.75 pounds
Weight of projectile, loaded and fused 1.97 pounds
Weight of one assembled clip of four (4) rounds 20 pounds

Twin mechanism with barrels and sight Mark 3 2300 pounds
Barrel assembly (including flash hider, water jacket, and recoil spring) 330 pounds
Gun Barrel only 200 pounds
Recoiling parts 490 pounds
Housing Assembly 130 pounds
Loader, complete with tray 210 pounds
Slide (twin) with elevating arc attached 800 pounds
Sight Mark 3 (for twin mount) 70 pounds
Sight Mark 4 (for quadruple mount) 90 pounds


1. Lubricants and Compounds Used:

a. Oil:

(1) Oil, Lubricating, Preservative, Light, O.S. 1362.

b. Grease:

(1) Bearing Grease for General Use in Naval Ordnance, O.S. 1350.

c. Cleaning Solvents:

(1) Diesel Fuel Oil, 7-0-2 (INT)

(2) Solvent (commercially known as Stoddard Solvent) P-S-661

(3) Kerosene-Water white 14-K-1

(4) Kerosene-VV-K-211

2. Daily

Oil the gun bore. Remove and disassemble the breech block, clean the parts, and coat with light film of oil. Clean and oil the inner cranks. Clean and oil the breech block guides in the housing.

3. After Firing

As soon as practicable after firing, depress the gun, clean, dry and coat the bore with a film of oil. Oil the breech mechanism in accordance with the daily schedule.


4. Weekly

Also, if the gun gets wet or dirty, or if there is any reason to expect corrosion to start.

a. Loader: Remove the loader and the rammer tray from the slide, and oil all loader sliding surfaces, springs and bearings. Use lubricant sparingly and exercise parts after lubricating. Coat all unpainted surfaces with a light film of oil or grease. Oil all working parts of the tray and coat all surfaces with a light film of oil or grease.

b. Housing: Remove and disassemble the housing. Clean all parts except the closing spring assembly with solvent, and coat all surfaces with a light film of oil. The breech block closing spring assembly is packed with grease and need not be disassembled. Use lubricant sparingly.

c. Slide: Oil all working parts. Apply a light film of oil or grease to all unpainted surfaces. Apply one shot with pressure gun at each of the six grease fittings after reassembly of mechanism.

d. Barrel: Clean, dry and coat the gun bore with light film of oil. Clean the barrel threads and coat with film of grease. Coat the outside surfaces of the water jacket and the recoil spring with a film of grease.

The following parts, if disassembled for repairs, should be repacked with grease at assembly:

Breech block closing spring
Feed rod plunger spring
Rammer spring
Rammer lever plunger spring
Star wheel journals, plungers and springs
Hand operating lever catches on slide
Trunnion bearings


To fill the recoil cylinder, elevate the gun to about twenty-five degrees, remove the fill plug, and unscrew the needle valve several turns. Using the filling funnel, pour the liquid in slowly until it overflows. Replace the plug and elevate and depress the gun slowly several times, returning the gun to


about twenty-five degrees elevation. Remove the fill plug, and using the funnel, pour in liquid until it again overflows. Replace the fill plug and reset the needle valve by closing it and then unscrewing it one-third of a turn. Approximately 1.2 pints of liquid is required to fill the recoil cylinder.


1. Elevating Arc

Elevating arcs have been manufactured by two different methods. Elevating arc (298728-1) is a casting, while elevating arc (345795) is a weldment.

2. Breech Block

In breech blocks of earlier manufacture, the firing hole bushing (298682-2) is threaded, and installed in the breech block by the use of the wrench (298869-6), and locked in place by the use of the locking pin (298681-2).

In later breech blocks, the firing hole bushing is an integral part of the breech block.

3. Loader

In loaders of later manufacture, no feed control thumb lever (298784-1) will be found. Consequently the instructions in Chapter VI to place this lever on the red arrow position are to be disregarded in operating these mechanisms.




Requests for copies of Ordnance Pamphlet No. 820 should be addressed to the nearest BuOrd Publications Distribution Center:

Commandant and Superintendent
U. S. Naval Gun Factory, Navy Yard
Washington 25, D. C.

Navy Yard
Mare Island, California

Officer in Charge
Ordnance Publications Subcenter
Naval Supply Depot
Pearl Harbor, T. H.

Commander Service Force, Seventh Fleet
Ordnance Publications Subcenter
c/o Fleet Post Office
San Francisco, California


Standard Navy Distribution List No. 15
2 copies each unless otherwise noted.

1. (1 copy), a-h, j-n, q-t, x, z-ff, ii-ll, nn, oo, qq, ss-zz; 2. (1 copy), d-h, n-p, s, t, v, bb, dd, gg; 3. a*, e*, f*, i*, j*, k*, 1*, r*, t*, x*, bb*, dd*, hh*, jj*, kk*, qq*, xx*, zz*, aaa,* ddd*, eee*; 4. a*, b*, e*, f*, g, i*, j*, k*, 1*, r*, t*, x*, bb*, dd*, k k*, qq*, tt*, uu*, vv, xx, yy*; 6. (3 copies), a-p; 7. a-c, f-i, j, 1 (Des Bases only), o, x, y, aa; 8. h (NOL and No. Beach only), i, j, n (SPECIAL LIST K, L, 0, S, V, Z, AA), q, r, u, gg; 8. (3 copies), x; 10. s, t; 11. a (Vice CNO, SecNav, ComdtMarCorps); 12.; 14. a.

* Applicable Ships.


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