To find out if it is truly out of round a chamber cast would need to be made, not just measure brass.If a camber is out of round, what would be the most likely cause?
That big company that uses a chambering machine where the reamer spins and barrel is stationary.
Someone touching up the chamber by hand.
Lots of ifs here but if the barrel was out of round or not concentric at the breech and the bore was not indicated before chambering started. That could make the breech wobble and the reamer cut on one side. Anything could happen but now most companies use floating reamer holders that allow the reamer to follow the bore making sure the chamber is concentric.