United States of America
37 mm/56 (1.46") M4
Updated 06 March 2009
Oldsmobile gun first used on P-39 airplanes.  Some of these guns were salvaged from crashed aircraft and installed on PT boats to give these small craft a "barge buster" weapon.  This proved successful and new guns were produced specifically for use on PT boats.

Action was long-recoil (Browning-type).  Used a vertically-sliding breech block and percussion firing.

Nomenclature Note:  This weapon was originally designated as the T9.  Designation changed to M4 about 1942.

Later in the war the more powerful 37 mm M9 was developed.  This weapon fired a larger, heavier round and a few were used on PT boats.

Unless otherwise noted, the data below is for the M4.
WNUS_37mm-56_m4_PT-631_pic.jpg

37 mm on bow of PT-631
Distinctive "arched" magazine was designed to fit into the nose of P-39 aircraft
US Navy Photograph taken in early 1945
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Gun Characteristics
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Designation 37 mm/56 M4
Ship Class Used On PT Boats
Date Of Design about 1937
Date In Service 1942 (on PT Boats)
Gun Weight Without breech:  55 lbs. (25 kg)
With breech:  213 lbs. (97 kg)
Gun Length oa 89 in
Bore Length about 81.6 in (207.2 cm)
Rifling Length N/A
Grooves N/A
Twist N/A
Length Of Rifling N/A
Chamber Volume N/A
Rate Of Fire Cyclic:  125 to 150 rounds per minute
Note:  Barrel length was 65 inches (165 cm).
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Ammunition
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Type Fixed
Weight of Complete Round N/A
Projectile Types and Weights
(see Note 1)		 HE - 1.34 lbs. (0.61 kg)
AP - 1.66 lbs. (0.75 kg)
Bursting Charge N/A
Projectile Length N/A
Cartridge Case Type, Size and Empty Weight Brass, 37 x 145R mm, N/A
Propellant Charge 0.15 lbs. (0.070 kg) NC
Muzzle Velocity HE - 2,000 fps (610 mps)
AP - 1,825 fps (556 mps)
Working Pressure 10.4 tons/in2 (1,638 kg/cm2)
Approximate Barrel Life N/A
Ammunition stowage per gun
(see Note 2)		 N/A
Notes:

1) It does not appear that PT boats used AP rounds.

2) Standard arched magazine held 30 rounds.
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Range
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Elevation With 1.34 lbs. HE Projectiles
Effective about 4,000 yards (3,660 m)
Maximum 8,875 yards (8,115 m)
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Mount / Turret Data
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Designation Modified 3-pdr pedestal type
Weight  N/A
Elevation N/A
Elevation Rate Manually operated, only
Train 360 degrees
Train Rate Manually operated, only
Gun recoil 9.5 in (24 cm)
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Data from
"Naval Weapons of World War Two" by John Campbell
"US Naval Weapons" by Norman Friedman
"Rapid Fire" by Anthony G. Williams
Off-site Resources

Pictures and data about 37 mm gun on PT-617 at Bill Maloney Website

Page History

09 November 2007 - Benchmark
06 March 2009 - Added recoil length