Round the neck of the stalk, just under the mushroom, is a
collar of asbestos, secured in a canvas cover; when the gun is fired,
the gas presses the mushroom against the asbestos collar, and squeezes
it against the walls of the bore. It is found that this cuts off all
escape.
We are at present using the Elswick method, which consists of a
flat-backed cup, abutting against the slightly rounded face of the
breech plug. The lips of the cup rest against a copper ring let in the
walls of the bore. On firing, the gas presses back the cup against the
rounded end of the breech-block, and thus forces the lips hard against
the copper ring.
It is difficult to compare the excellence of these various systems, so
much depends on the care of the gunners, and the nicety of manufacture.
The German and French marine methods permit the parts to be quickly
exchanged when worn, but it is necessary to cut deeply into the walls
of the gun, and to make the wedge, or breech-screw, considerably larger
than the opening into the chamber.
The Elswick plan is decidedly better in this last respect, but it
requires several hours to extract and renew the copper ring where worn.
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