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)


"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 - Benchmark
06 March 2009 - Added recoil length
30 June 2024 - Converted to HTML 5 format