Welin Stepped Interrupted Screw Breech
Britain, France and the United States preferred screw breeches for most calibres, but the major drawback of the de Bange interrupted screw as first implemented was that only half of the circumference of the breech block could be threaded, and hence it needed to be fairly long to achieve safe locking of the breech, which required three separate motions to open after firing : rotate to unscrew, retract the screw and swing to the side. Loading required the 3 operations in reverse. This was hence termed a "three motion block" and was slow to operate. Axel Welin solved this problem in 1889-1890 with his stepped interrupted screw Welin breech block. This had threads in sets of steps of increasing diameter so that instead of only half of the screw being threaded, the fraction threaded was number of steps / (1 + number of steps) : i.e. if the block of a large gun had four steps of threads, 80% of the screw was threaded, allowing a much shorter screw and hence breech block. This allowed the block to be unscrewed and swung out in two motions : the "two motion interrupted screw" breech.
Further improvements such as that of Smith-Asbury in 1916 allowed the whole action of unscrewing and swinging aside of the breech screw to be accomplished in a single continuous motion, resulting in the "single-motion interrupted screw breech" still in use today.
Read more about this topic: Rifled Breech Loader
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