Raimey,
The rifle barrel had been drilled out with a twist drill ( after being opened up with a piloted counterbore, to keep the drill flutes from catching on the extractor cut), because the rifle chamber was too tapered for a hand reamer. The shotgun barrels had been opened with reamers, starting with a size to fit into the shotgun chamber, and stepping up to one sized to clear the new shotgun tube. Since the rifle barrel had been drilled, the hole needed to be reamed, to adjust it to correct size to clear the new rifle barrel shank. The reamer, being used is a common adjustable reamer, adjusted to size. This type reamer was possible, because the barrels had been cut off and it was a "through hole". Note that the barrels had been drilled/reamed before being cut off. By turning the reamer(or drill)in the headstock of a "speed lathe", and centering the barrel being worked on with the tail stock, while "dogging" the barrels with a hand vise against the lathe "way"; the original "convergence" of the barrels was maintained. This way allowed the work to be done, without more sophisticated equipment, and time consuming "set up".
Mike
