The brass body of the primer is 0.56 inch long and has a diameter of 0.52 inches at the screw-threaded portion. Three equo-spaced recesses for the primer key are formed round the circumference of the base. The model number, C/13, followed by the letters "nA" indicating new pattern, is stamped in the base.

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 0.33 grain filling consisting of mercury fulminate 52 per cent, potassium chlorate 23 per cent, antimony sulphide 19.7 per cent, and grit 5.3 per cent. The composition is sealed by a covering of varnish without the addition of a foil disc.

The brass anvil plug is screwed into the body and is shaped at the base to receive the cap. A flash channel is formed through the centre of the plug.

The magazine formed in the upper part of the body contains 8.7 grains of pressed gun-powder covered by a paper disc. The magazine is closed by a brass washer covered by a plastic disc. The disc is secured by the rim of the body being turned inwards.