With a M1A, if a short chamber is cut, the only way I know to finish hand cut the chamber is with a special reamer from Clymer that has long steel rod that is feed from the muzzle that then threads into the front of the reamer, then the reamer is pulled foreword with a T handle on the end of the steel rod sticking out of the muzzle end. The reason for this elaborate, and expensive reamer system is once the barrel is installed in the receiver, a conventional reamer can’t be used because of the receiver design. I never had done this, but Clymer is in my neck of the woods, and over the years my shooting buddy and I had visited Clymer many of times to either pick up a reamer, or headspace gauges, or just to see what wildcat cartridges people had dreamt up, and remember asking about their M1A chamber finish reamer, and it was demonstrated for us. Just something to consider. Brownells has this.These are screenshots of the reamer system.