[Tom2]

Without machine tools, you are pretty much limited to working the front of the frame, I guess. If you work it down off-square, all that will result is a gap on one or more sides. And the replicas have a reputation for being soft steels, they are not probably hardened, like modern guns that need to hold high smokeless pressures. All I can say is, maybe work hard to keep it clean and square. Taking off a few thousandths, maybe that would only require some sanding, but I am at a loss how to measure it. Turning in would reduce your cylinder to breach gap, but a small fraction of a turn to square things up, might not make much diff. Or require much material removal.