One mechanical phenomenon rarely, if ever, discussed in barrel fit is the Poisson effect. It concerns the behavior of thick wall cylinders (walls thicker than 1/10 of the diameter). Under pressure such a cylinder shortens and expands radially. Presumably that happens to the chamber end of barrels under pressure and this is followed by an equally fast recovery. This process must involve some considerable slamming of chamber ends against the breech face and the hook on the cross pin, and all this is in addition to the flexing of the action bar.
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