US Primer, Percussion, M22, M22A1, M22A2, M22A3
This primer is one of the primers used with the reduced and normal charges for the 75mm guns. It was originally designed to accommodate over 800 grains of black powder, but it was found that better performance could be obtained in some rounds with a primer of the same length, but with reduced charge. The reduced charge is held in the end of the primer tube adjacent to the primer head by means of a cardboard cup. The empty portion of the tube is lined on the inside with a paper liner. The paper liner serves a dual purpose: first, to prevent the grains of propellent powder from entering the upper end of the primer tube; and second, it also serves to support the black powder charge retaining cup.
The head assembly of this primer is similar to that of the Primer M83. To it is threaded a brass tubing body, closed at the outer end by a threaded brass plug. A 75 grain charge of black powder is contained in the lower end of this flash tube as described above.
This primer has been redesigned on three different occasions. The A1 modification is similar to that of the M23A1 Primer, consisting of a redesign of the primer head.
The A2 modification consists of a taper being introduced into the head of the primer beyond the flash hole and a resultant reduction of the auxiliary charge of black powder from 75 grains to 65 grains. This was done to eliminate wastage of the black powder in the auxiliary charge. Previous to this modification the powder in the periphery of the primer head immediately behind the flash hole was not ignited 'directly by the flame of the primer mixture (which could not reach it), but indirectly by flame from that part of the auxiliary charge ignited by the primer mixture.
The A3 modification of the primer is similar to that of the M23A2 Primer, consisting of a lengthening of the firing plug.