In the '60's, with $15 milsurps being sporterized by the 1000's, the process was to hacksaw the barrel off, use a file and square the muzzle.
Then put a plug of cotton down the barrel, take a brass round head stove bolt and some medium and fine valve lapping compound, chuck the bolt in a hand or electric drill holder, yes, we still used hand crank drill holders then, and lap until a nice ,clean crown was made, clean up and go shoot.