The brass body of the primer is 0.55 inch long and has a diameter of approximately 0.5 inch at the screw-threaded protion. Two keys flats are formed in the base, which is stamped with the model number C/33.

The brass cap is positioned inside the base of the body, under the anvil, and is not visib-le from the exterior. The cap contains a 0.37 grain filling consisting of mercury fulminate 24.6 per cent, potassium chlorate 37.6 per cent, antimony sulphide 29.6 per cent and ground glass 8.2 per cent. The filling is covered by a disc of tinfoil without varnish.

The anvil plug is of brass and is screwed into the body over the cap. A screw slot is cut in its font face, and two slots are formed, diametrically opposite, down the side to provi-de flash channels.

The magazine formed in the upper part of the body contains a 2 grain filling of granular gunpowder immediately above the anvil plug over which is a 6 grain pellet of gunpowder. The pellet is covered by a paper disc and the magazine closed by a plastic disc inserted over a brass washer. The closing disc is held by the turned-in rim of the body.