It is a screw that has a section of thread along its axis removed. The screw is mated with a hole at the rear of the weapon's chamber; i.e., where the screw has threads the hole does not, and vice versa. The screw can thus be smoothly inserted all the way into the gun, after which as little as 1⁄6 of a turn can engage the two sets of threads securely, sealing the breech. Interrupted screws are occasionally seen in loose gunpowder rifles, as this mechanism was historically one of the few practical ways to achieve a gas-proof seal with a breech-loading firearm that does not employ cartridges. An earlier method was the use of a wedge to block the rear of the gun.
Sealing was further improved with the de Bange obturator in 1872.
The major weakness of the original designs was that only half of the circumference of the breechblock could be threaded, hence a fairly long breechblock was still required to achieve a secure lock. Axel Welin solved this problem with his stepped interrupted screw design : the Welin breech block of 1890. This design has threads of the block and breech cut in steps of successively larger radius. This allows e.g. a breechblock with 4 steps to allow 4/5 of the block circumference to be threaded, allowing for a much shorter breechblock while still only requiring 1/5 turn to open or close. This is the basic design still in use with bagged charge artillery.
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