All the american .30s are .308 bullet diameter, whereas 7.92x57 (8mm Mauser and others), are .323. There's no way to simply rechamber to a smaller bore unless you want to have the existing barrel reamed, then put a smaller caliber liner in the barrel. I can see the advantages of this if you want a "sleeper" (a factory stock looking rifle that performs much better by having a match quality custom barrel liner). Otherwise it's not really practical since by the time you buy a barrel liner, reaming the old barrel to the larger liner inside diameter for the liner, then installing the liner/& anchoring it, you'll usually be out at least twice what you would have in a barrel right off the bat.