Just a little for those that aren't familiar with machine tools, reamers are available in oversize increments for pin fitting (not necessarily a gun term pin). It's not uncommon to see .0005" oversize increments for a .001-.002" depending on maker. Also, the opposite, smaller, is common.
Rotating parts in a sxs gun are commonly casehardened. I would go along with Miller's suggestion that selecting a .0005" oversize reamer will provide sufficient clearance. However, after casehardening there will likely be some shrinkage of the hole. You don't run a reamer into a casehardened part if you want the reamer to stay sharp. A brass lap with lapping compound would clean up such a hole and leave it free of the relatively loose surface oxides that may be abrasive.
