Quote:
Originally Posted by LugerVern
There are some other forces taking place we are not considering.
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Indeed. There is heat, for instance. Not just the heat of combustion of the propellant, but heat of friction. A quick firing of a full magazine may induce enough heat to loosen the threads...
(I know; heat itself is not a force, but the expansion & contraction it induces can be so considered...)
And there is Newton's 3rd law of motion. There is a constant rotational pressure forcing the barrel into the receiver as the bullet travels forward; Newton's law tells us that when the pressure is suddenly relieved, there will be a reversal of pressure in the opposite direction - the "equal and opposite reaction" (actually, an oscillation of opposing pressures until they gradually die out)...
Since the pressure reversal is the last force acting on the barrel (the bullet has exited), and the heat is still present, it could be argued that the combination of these two
might impart an unscrewing action on the barrel...
I've never seen or even heard of a barrel of any pistol/rifle unscrewing from use (assuming it was properly machined, fit, and torqued in the first place), and I'll maintain a level of skepticism in this regard...
(Although you can see from the progression of S&W revolvers that S&W must have had some reason for pinning their later model barrels...)