RE: back bored Moisin/nagant
Mosin nagants tend to be very inconsistantly made as well. They are supposed to have a nominal groove diameter of .310" but I have one that is .315." Amazingly enough, the one rifle that shoots better than the rest has a groove diameter of .314" has a rough bore and isn't counterbored. If I get one that shoots good out of the box, I shoot it, if it doesn't shoot well, I don't even waste my time on it. Mausers I will tinker with though.
If you have rifles that aren't counterbored, you can touch up the crown very easily if the rifling looks badly worn. Take a cariage bolt, chuck it up in a drill, apply valve grinding paste to the bolt head and grind away at the muzzle until it has a dished out appearance. Take a file and dress the outside edges of the crown if you wish. Best to plug the muzzle with a rag or earplug to keep the grit out of the bore.
If you wqant to cut a bit off the barrel, use a pipe cutter to cut until just short of breaking through to the bore, then finish the job with a hacksaw. Use the carriage bolt/grinding past to cut a new crown.