The copper of the driving band is engraved and pressed by the lands into the grooves as the projectile proceeds. To avoid copper deposition in the bore, which would reduce its diameter and affect the form of grooves, tinfoil is combined with the propellent.
A breech mechanism is a mechanical device for closing the rear end of the chamber of a gun. It includes the mechanism for firing the gun, and safety arrangements for preventing the gun from being fired before the breech is fully locked. It also em braces the necessary operating gear which may be "hand" or "power." If the latter, "hand" alternative is generally provided which can be readily connected or clutched in. The chief require ment is safety, with ease and rapidity of operation. Two main types of mechanism prevail:—(i) The "screw plug" with inter rupted threads, held in a carrier hinged to the gun, so that the action is a swinging and rotating one. This "swinging" type may be used for both B.L. and Q.F. guns. (a) The vertical or hori zontal sliding wedge shaped block, travelling in a mortice across the rear face of the gun. This "sliding" type is only suitable for a Q.F. gun. Other types are:—The sliding and rotary combina tion, the rotating eccentric block and the sliding bolt. Certain automatic and machine guns employ a "breech action" in which the operations of opening, extracting, reloading and closing are effected by the recoil and counter-recoil motion augmented by springs.
B.L. Mechanism.—For B.L. guns (in which the charge is bagged) the breech screw is fitted with an obturator for sealing the escape of gas to the rear. It requires a channel or vent to convey the flash from the tube to the charge to fire the gun.
The illustrations (figs. 13 and 14) of an Asbury "hand" and "power" mechanism show an example of a modern screw plug type. The screw is of the Welin form, an arrangement in which the surface is divided circumferentially into a number of seg ments of different radii. The number of plain segments is less than half the total number, thus giving an increased bearing sur face compared with a plain cylindrical screw with interrupted threads. The Welin compared with the cylindrical screw allows of
equal strength with a reduced length and weight of screw. The reduced length enables the screw to be swung into the breech opening instead of having to be first swung and then entered in a separate movement. Direct swing entry can also be obtained by a breech screw in which the rear portion is cylindrical and the front conical.