Ammunition - 20mm

Twenty-millimeter ammunition is the ammo used on the M197 automatic gun. The rounds are 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 20-mm projectiles are of comparable weight.

Muzzle velocity for the types of 20mm ammunition discussed below averages 3,380 feet per second. The table below shows the approximate ballistic drop of the 20mm ammunition.

Approximate time of flight* (range (m) / seconds)	Approximate ballistic drop* (range (m) / mils)
1,000 / 1.5	1,000 / 9
1,500 / 3.0	1,500 / 21
2,000 / 5.0	2,000 / 42
*M56 HEI fired from IGE hover.

Maximum effective range of the 20mm system on the AH-1 is normally listed as 2,000 meters. Fuze 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. A more realistic maximum effective range is approximately 1,500 meters; however, this depends on the type of target that is engaged. Rounds with an R50 value, which means that there is a 50 percent chance of the round penetrating rolled homogeneous armor at the given conditions / range, are as follow.

Hull and turret thicknesses of selected armored vehicles
Armored Vehicle	Hull	Turret
BTR-70	.40 in (10mm)	.28 in (7mm)
BRDM-2	.56 in (14mm)	.28 in (7mm)
BMP	.76 in (19mm)	.92 in (23mm)
BMD	.60 in (15mm)	1.00 in (25mm)
ZSU-23-4	.37 in (9+mm)	.35 in (9mm)

M55A2 Target-practice. The M55A2 TP (diagram) 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.

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

M56A3/A4 High-explosive incendiary. 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 causalities to exposed personnel within a ± 2 meter radius. It has a base plate which prevents ignition of the incendiary mixture by propellant gases. The M56A3/A4 (diagram) is assembled with a single-action M503A3 point-detonating fuze. The explosive charge is 165 grains (.37 ounces); the incendiary charge is 20 grains. The HE mix and the incendiary mix are combined into one pellet in the A3 HEI. To improve the fire-start capability of the A4, the incendiary pellet is inserted into the projectile and then the HE pellet is added.

M242/M242A1 High-explosive incendiary-tracer. Except for the addition of a tracer element, the M242 / M242A1 HEI-T is basically the same structurally and functionally as the M56A3 / A4.

M53 Armor-piercing incendiary. The M53 API (diagram) 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 explosive charge is 65 grains (.14 ounce).

M246 / M246A1 High-explosive incendiary with tracer and self-destruct feature. 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 M503A3 point detonating fuze. The tracer burns for about 5 seconds whereupon the relay charge ignites and detonates the HEI charge low order. If impact with the target occurs before self-destructing, the PD fuze causes the HEI charge to detonate high order. The M246 has the HE and incendiary mix combined as one pellet; the M264A1 has the HE and incendiary charge loaded as separate pellets.

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

20-Millimeter