Performance of 20mm Ablative Ammunition

M55 A2, 20mm ammunition was modified by placing five cubic centimeters of gelled dimethyl silicone compound between the propellant charge and the projectile. The function of the silicone was to reduce barrel heating and erosion by coating the bore surface, forming an insulative and ablative shield against the hot propellant gases. Several thousand silicone-modified and standard rounds were fired in thermocouple-instrumented M39 and M61 Vulcan cannons. Very substantial reductions in heat input to the revolving drum of the M39 and the rear portion of the M61 barrel were effected, and cook-off analyses indicated a 100-percent increases in the burst length safe against cook-off in both the M39 and the M61. Erosion reduction and yaw-life increase were demonstrated in 250-round burst firing of the M39. In the M61 barrels, which were unplated, heavy coppering deposits with the modified ammunition and barrel bending interfered with rational erosion testing, but bore enlargement near the origin of rifling was decreased by the modified ammunition. Eroded and coppered barrels were sectioned and examined by metallographic and electron microprobe techniques. Ballistic performance was maintained in the modified ammunition and no failures or gun stoppages attributable to the ammunition were experienced.