Twenty millimeter ammunition is electrically primed; chamber pressure varies from 60,500 to 61,500 psi. Projectile weight of the M56 HEI round is 1,543 grains (3.5 ounces); other 20mm projectiles are of comparable weight. Muzzle velocity for the types of 20mm ammunition discussed below averages 3,380 feet per second. Types of 20mm munitions available are discussed below and shown in Figure 5-3.

(1) Target practice M55A2. M55A2 TP ammunition is used for gunnery training and test firing in lieu of the service round. It has a hollow cavity projectile body without a fuze (inert). The nose of the round is constructed of aluminum and is swaged to the projectile body.

(2) Target practice tracer M220. Except for the addition of a tracer element, the M220 TPT is very similar physically and ballistically to the M55A2. Tracer burnout usually occurs at a range of approximately 1,500 meters (±100 meters).

(3) High explosive incendiary M56A3/A4. Functioning with both explosive and incendiary effect, the M56A3/A4 HEI is intended for use against ground targets, including lightly armored vehicles. This thin walled, steel projectile can produce casualties to exposed personnel within 2-meter radius. It has a base plate that prevents ignition of the incendiary mixture by propellant gases. The M56A3/A4 is assembled with a single action M505A3 point detonating fuze. The explosive charge is 165 grains (.37 ounce); the incendiary charge is 20 grains. The HE mix and the incendiary mix are combined into one pellet in the A3 HEI.

(4) Armor piercing incendiary M53. The M53 API is intended for use against lightly armored targets. It functions with a combined incendiary and has a penetrating effect. The body of the projectile is constructed of solid steel; the nose is constructed of an aluminum alloy. The incendiary charge is 65 grains (.14 ounce).

(5) High explosive incendiary with tracer and self-destruct feature (M246/M246A1). The M246/M246A1 HEI-T-SD is intended for use against aerial targets. It has an HEI charge, a self-destruct relay charge, and a tracer element. It is assembled with an M505A3 point detonating fuze. The tracer burns for about 5 seconds whereupon the relay charge ignites and detonates the HEI charge. If impact with the target occurs before self-destruction, the PD fuze causes the HEI charge to detonate. The M246 has the HE and incendiary mix combined as one pellet; the M246A1 has the HE and incendiary charge loaded as separate pellets.

(6) Dummy (M51A2/XM254). The M51A2 is an inert round of solid metal construction and is used for nonfiring system loading and system checkout. The XM254 is constructed of plastic. As with the M51A2, the M254 also reduces wear on gun components and feed mechanisms.

b. Twenty millimeter fuse functioning and penetration are affected by velocity and angle of impact at all ranges, particularly at ranges in the upper one third of the 2,000 meter value. However, this depends on the type of target that is engaged. Rounds with an R50 value, a 50-percent chance penetrating rolled homogeneous armor at the given condition and range, are as follows:

• M56 HEI: .25 inch (6.3 mm), RHA at 60 degrees, obliquity at 221 meters; .50 inch (12.5 mm), RHA at 0 degrees, obliquity at 104 meters.

• M53 API: .25 inch (6.3 mm), RHA at 0 degrees, obliquity at 1,000 meters.

• M940 MPT-SD: .25 inch (6.3 mm), RHA at 60 degrees, obliquity at 940 meters; .50 inch (12.5 mm), RHA at 0 degrees, obliquity at 518 meters.