No1Mk111 Receiver Holding Fixture

[Bruce_in_Oz]

Are you using something like a Bridgeport or a milling attachment on a lathe?

If the latter, and you don't want to remove the barrel, you "could" grab the muzzle in a 4-jawed chuck and have a LONG mandrel / bar that runs through the body and into the chamber and which has a "centre" hole bored in the "rear' end..

Depends on exactly what sort of milling you are contemplating.

Alternately, align the barrel / body assembly between suitable centres and then attach a couple of "cats" which, after centering, will run in fixed steadies. Use a face-plate and dog to drive the barrel and a flat plate attached to one of those nifty "convertible "live' centres to stop it drifting about.

One way to minimise movement during milling is to take a lot of very small, very fast cuts: I have used a high-speed (30,000 RPM-ish) die grinder and Tungsten Carbide cutters / burrs. Requires minuscule but CONSTANT feed force, is VERY noisy, but if the workpiece is locked up solid and the grinder is also firmly mounted, the process leaves a very clean finish. MANY years ago, I did a batch of "conversions" to several scruffy Garands; sleeving and re-cutting the chamber (alternately setting the barrel right back and re-chambering / threading), opening the bodies up to take M-14 mags, cutting away the mag bottom in the trigger housing, shortening the op rod, etc. etc, not unlike how the Italians (and Indonesians) made the BM-59. 1/4" carbide tools running at 30,000 RPM sliced through the heat-treated 8620 steel like it was butter. Cutter "chatter" was invariably fatal to the Carbide tools.