I’ve done a lot of chambers, mostly in relined barrels, mounted vertically in a vise with the reamer in a big tap wrench. Biggest difference I can recall offhand between bore and shell diameter (without preliminary drilling to remove stock) was a .25-35. A big sharp-shouldered Magnum would be a lathe operation, no doubt; too arduous otherwise.
Brush Rigid cutting oil (or Cool Tool) on the flutes, insert reamer, pull down slightly while rotating the wrench, count eight half turns, “feather” the last turn while withdrawing the reamer, brush off chips; re-oil; a cleaning rod with patch down the bore (trash bag underneath); insert reamer and repeat.
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Every now and then, I loosen the vise jaws, turn the barrel 45 or 90 degrees and retighten for the next few rounds of wrench turning. An octagon barrel profile helps here, but a round one can be done close enough. I use wood, copper or aluminum jaws in the vise.
Eventually, the reamer is in far enough so I can start checking headspace. I’ll screw on the action at this point so I can close bolt or block on the gauge or cartridge rim. I have homemade extensions for reamer and wrench that I use at this point. Then it’s fewer partial turns, less downward pressure, more barrel rotation in the vise, and more cleaning and checking between reams.
Eventually, it’s done, and however long it took, it’s nothing to the time I’ll be shooting it. I’ve never seen bulges or asymmetry in fired cartridges that would indicate that I’ve induced swelling or runout in the chamber by reaming this way.
And I have to say, I’ve gotten pretty cynical about some of these claims about precision manufacture. One guy in Precision Shooting (IIRC) claimed his gunsmith-to-the-stars did his barrel work “to a millionth of an inch.” But his description of the process never mentioned the other person of identical size, weight and body temperature that I heard had to stand the same distance behind the lathe as the operator was in front in order to truly effect this precision. I think he may have been appealing to the Voodoo Element there.