Seems that normal was HE rounds....From Wikipedia..... so consider the source:
http://en.wikipedia.org/wiki/M4_cannonM4 cannon
37 mm Automatic Gun, M4
Type
Autocannon
Production history
Designer John Browning - Designed 1921-1938
Manufacturer Colt
Produced 1939
Specifications
Weight
213 lb (97 kg)
Length
89.5 in (2.27 m)
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Shell
37x145mmR M4
Caliber
37 mm (1.46 in)
Action
recoil operation
Recoil
9⅝ in (245 mm)
Rate of fire
150 rpm
Muzzle velocity
2,000 ft/s (610 m/s)
Feed system
30 round magazine
The 37 mm Automatic Gun, M4, known as the T9 during development, was a 37 mm (1.46 in) recoil operated autocannon designed by Browning Arms Company.[1] The weapon, which was built by Colt, entered service in 1942. It was used in the Bell P-39 Airacobra and P-63 Kingcobra.
Designed primarily as an anti-aircraft weapon, the gun had a muzzle velocity of 2,000 ft/s (610 m/s) and a cyclic rate of 150 rounds per minute. It was normally loaded with high-explosive shells, but could also be loaded with the M80 armor-piercing shell which could penetrate 1 in (25 mm) of armor plate at 500 yd (460 m). It was magazine-fed and could be fired manually or by remote control through a solenoid mounted on the rear of the gun.
Recoil and counter-recoil were controlled hydraulically by means of a piston and spring combination connected to the recoiling mechanism and operating in an oil-filled recuperator cylinder mounted to the stationary trunnion block assembly. The recoiling mechanism of the gun included the tube and tube extension, recuperator piston and piston rod, lock frame assembly, driving spring assemblies, and the breechblock assembly. The nonrecoiling parts included the trunnion block group, the feed box and feeding mechanism, the recuperator cylinder and bushing, the back plate group, and the manual charger assembly