The Whitworth breechloader, with its peculiar hexagonal bore, used a device resembling a hinged screw cap. The cartridge was in a hexagonally-shaped tin container which assisted in preventing the escape of gas from the breech. A lubricating wad (tallow and wax was used) was inserted behind the projectile. The projectile was a mechanical fit and did not rely on expansion to make a tight fit in the bore. Because of this, and because there was no chamber, by closing the breech and using a copper disc as a gas check, the guns could be (and often were) used as muzzle-loaders. This could not be done in the Armstrong, with its lead-coated shell and powder chamber larger than bore size.