Squeeze bore

These weapons are used in conjunction with special sub-caliber ammunition where the projectile is fitted with soft-metal flanges which fills out the caliber.

Rather than squeezing solid shot, this is accomplished through a hardened penetrator core (tungsten for example) and a softer outer jacket (aluminium alloy) forming flanges or wings.

[2] The squeeze bore concept was first patented by German inventor Carl Puff in 1903[7][8], even though the general principle was known already in 19th century and later applied in lighter fashion on Armstrong guns, on which only the muzzle yet not the barrel itself was of slightly smaller diameter (to cast off the sealing leather-bag, the powder-cartridge was infolded into).

Later, Hermann Gerlich in the 1920s and 1930s experimented with the concept resulting in an experimental 7cm anti-tank rifle with a 1,800 m/s (5,900 ft/s) muzzle velocity.

[2][9][10] Other uses of the squeeze bore include the British Littlejohn adaptor, a QF 6-pounder adapter and a version of the M2 machine gun produced during WW2.