A bullet sizes up or down to fit the bore. It's done in a thousandth part of a second. Read P.O. Ackley's experiments firing .35 caliber bullets out of a .30-06.
A bullet stopped in the bore causes an issue when another bullet is fired up behind it. The bullet coming forward is compressing the air and the slug stopped in the bore needs high pressure to get it moving. The shockwave coming to the base of the stuck bullet causes the bullet to swell up, bulging the barrel. Then it moves and is sized down again and goes on out the bore. It could split a thin barrel I am sure.
This is an 1860 Army .44 barrel that had a stuck round ball in the bore. A full load was fired after the squib load because the shooter did not realize the previous shot had not exited. It bulged the barrel but did not hurt the accuracy. Just looks ugly.
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I've seen .45 Colts and .45 ACP 1911's with full power loads do the same thing ... and .357 Magnums. The pistol was not harmed other than needing a new barrel.
But if you fire an oversize bullet down a bore .. rifled or smoothbore .. with nothing in front of it, it will size down quite a bit with no issues. The .300 Blackout fired in a 5.56 turned the bullet into a very long .22 bullet. I don't recommend it but as far as I know, no guns were destroyed.
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WHAT WRECKS THE FIREARMS USING OVERSIZE BULLETS IS IF THE CARTRIDGE DOES NOT SEAL THE CHAMBER TO KEEP THE EXPANDING GAS FROM RUSHING BACK THROUGH THE ACTION. IF THE GAS CAN COME BACK THROUGH THE ACTION IT CAN DESTROY THE ACTION OF THE FIREARM.
Again ... read P.O. Ackley's laboratory tests on oversize bullets in rifles.
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