Breech closure was effected by a vertical sliding block of rectangular section called a vent piece. Obturation was achieved by a conical copper-ringed plug on the face of the vent piece being forced into a copper bush at the mouth of the chamber shaped to receive it, rather like a cork in a bottle. Plug and ring were held tightly together by a hollow breech screw through which the gun was loaded. Both the rear end of the breech screw and its handle were fitted with tappets to assist in tightening the assembly.

To load, the vent piece was lifted clear. The projectile was then rammed through the breech screw until stopped by the shot seating, followed by the cartridge. The vent piece was then replaced and the breech screw tightened, after which a friction tube was inserted in the vent. Lastly, a lanyard was hooked to the ring on the tube.