Setting up the barrel between centers is asking for trouble. How would one indicate the bore with a center stuffed in it? I’ve threaded piles of barrels and each and every one had the bore dialed in in a 4 jaw chuck.
More than likely you’ll need to thread them 1/2x28 and use a bushing style adaptor like Flintlocke said. For calibers under .277 and really skinny barrels I’ve threaded them 7/16”x 24 or whatever the NF pitch is for 7/16 threads. You would of course need to have that bushing custom made more than likely but it’s an easy enough job.
