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"A device for preventing the escape of gas through the vent or around the breech-mechanism which closes the rear end of the bore or chamber of any breech-loading small-arm, machine-gun, or cannon. Freire Gas-check. AA, breech-block; BB, expanding bolt and bolt-head; CC, expanding steel ring or gas-check; S, spiral spring; D, check-nut and set-screw." -Whitney, 1911

Freire Gas Check

"A device for preventing the escape of gas through the vent or around the breech-mechanism which closes…