Try using full form inserts to thread with. You will need a different insert for 24TPI versus 16 or 18 TPI, as the insert is set to cut a full profile and each TPI is different depth.
I turn the tenon to size ( machinists handbook is your friend, unless it is a trued, not standard thread like in a trued action. I hit it with a light coat of red dykem, while that is drying I set the stops on the carriage. Take a light .003 pass, double check the thread count with a gauge, then make progressive passes until dykem is gone, double checking against the infeed to determine when I get to full depth. I will usually make one spring cut, after I arrive where I should be, put a small chamfer on the starting side,
Clean up with brakleen, then use a thread file to dress any remaining burrs. SInce I almost use exclusively stainless barrels and lots of SS actions, any burr is a Bad Thing. Run the action on with a little light oil like 3 in 1, check for looseness. If I get any resistance whatever I back off check for burrs etc. After you've done a few, you can feel in the first two rounds whether its right or tight. If it's tight, it gets another light cut, cleaned and de-burred again.
If it is for an action that is glued in like a BR rifle, it isn't as easy to check that fit. That is usually when the thread wires come out, and the old barrel becomes the test piece for the new. I keep a sheet on every action/barrel I do, with the as left tenon dimensions, so that I can fit a new barrel , without having the action in hand.
I have to imagine most of the guys doing large numbers of rifles, like GAP or LR are using either a standard system and dimensions or have very good records to refer to.
I'll try to get a closeup of a full form insert later today and upload that, if you haven't seen one before. I like them, and it makes really good threads, although I also have some HSS ones also.
