A common method during World War Two, often called the Gerlich-, Littlejohn- or tapered bore principle, was to fit sub-caliber ammunition with soft metal flanges filling out the missing caliber and then fire them from squeeze bore barrels. Squeeze bore barrels, often found on shotguns (see Choke (firearms)), progressively decreases its bore diameter towards the muzzle, resulting in a reduced final bore. When fired the flanges on the projectile would fold inwards as it travels through the reducing inner diameter of the squeeze bore.[1]