The usual proceedure for M1icon rifle is to use a pullthrough ream so when the bolt closes, the headspace's correct. The gunsmith may know what he's doing, but the conventional ream is pullthrough. I did do mine the way your gunsmith says but most of these others here will disagree with me. If you have to ream, it's going to be very slight. Yes, you're extending the chamber when reaming. Also maybe removing the slightest amount from the shoulders. The pullthrough reamers don't really cut at the back end.

Does that answer you?