US Primer, Percussion, Mk 2A1 (Mk IIA1)
The body of the Mk. IIA1 21 grain Percussion Primer is 1.595 inches long and the diameter, except for the flange at the head, tapers from 0.356 inch at the head to 0.348 inch at the open end for press fitting into the cartridge case. The primer body is made of cartridge brass. The percussion assembly consisting of anvil, paper disc, percussion compound, and percussion cup is inserted into the flanged head of the primer in the order given. The primer charge of 21 grains of Army black powder. Grade A4, glazed, is put in through the forward end of the primer and consolidated into a definite space by sealing it with onionskin paper, beeswax, and shellac. The beeswax must be a minimum of 0.11 inch in thickness.