The primer body is of brass with a flange at the base, and is screw-threaded externally for insertion in the cartridge case. The diameter of the threaded portion is approximately 0.55 inch. The interior of the body has a small recess near the base to accommodate the initiator capsule and is screw-threaded to receive the magazine.

The initiator capsule is of copper and contains a brass anvil with a conical flash-hole and 0.86 grain of initiator composition. The composition consists of mercury fulminate 31.9 per cent, potassium chlorate 32.4 per cent, antimony sulphide 32.6 per cent, and ground glass 8.1 per cent. The composition is protected by varnish, but there is no foil disc.

The brass magazine is in the form of a cup with a flash-hole in the base and an external screw-thread for assembly in the body. The magazine contains a 14.2 grain pellet of gunpowder with a central perforation.

The primer is closed by a brass disc over which the mouth of the body is turned and the closing sealed with varnish.

When the base of the primer is struck centrally the initiator composition is crushed on the anvil and initiated by mechanical shock. The flash produced by the composition pas-ses through the flash-hole in the anvil and the magazine and ignites the powder pellet. The explosion of the powder pellet blows out the closing disc and ignites the propellant charge. The escape of propellant gases over the primer is prevented by the expansion of the body in the primer boss. Internal sealing is provided by the base of the primer.