If you are referring to the protrusion from the bottom of the barrel that holds the forend in place, by way of a spring loaded mechanism, it is called the barrel lug. I can't recall any other name for it. It replaced the barrel loop, through which the wedge passed, and which held the forend in place on very early guns, even muzzleloaders.

Not sure if the English use that term or something else.

SRH

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