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.

Gun Characteristics

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)
Barrel Length 65 inches (165 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


Type Fixed
Weight of Complete Round N/A
Projectile Types and Weights HE: 1.34 lbs. (0.61 kg)
AP 1: 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 N/A 2
  • ^It does not appear that PT boats used AP rounds.
  • ^Standard arched magazine held 30 rounds.


Range with 1.34 lbs. HE
Elevation Range
Effective about 4,000 yards (3,660 m)
Maximum 8,875 yards (8,115 m)

Mount/Turret Data

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)


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

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

Page History

09 November 2007
06 March 2009
Added recoil length