US Cartridge, 37mm Practice, Mk 2, Mk 2A1
This round was constructed to simulate the high explosive shell for practice work. It is identical with the high explosive round except that the bursting charge consists of 0.05 pound of black powder instead of 0.053 pound of TNT; and that Fuze, Base Practice, M38 (a percussion type of fuze), is used instead of Fuze, Base Detonating, M38A2. A percussion fuze is necessary to ignite the black powder charge.
The A1 modification of the Mk II Practice Shell was the addition of 15 percent colloidal graphite to the black powder charge. The Mk II Practice Round could not be fired over personnel due to the possibility of premature explosion. The adding of colloidal graphite was for the purpose of eliminating this possibility of premature explosion of the bursting charge.