Relatively difficult depending on what tooling you have. You would need to make a reamer basically to cut the internals due to the small size. It needs to be a exact size and a high grade finish or it won’t work.Given you have a lathe and make screws, how difficult would it be to make your own sizing die?
Pm me the link, could not find it.There's a set of RCBS does on that well know auction site.
It would be a option, you would have to have the hardening capabilities though as well as some hardenable steel.A neck sizing die is easy to make.
"D" style reamers were something gunsmiths made in years past. I've made a few. They cut slowly compared to a fluted reamer, but they do work. Could that be an option for a sizing die?
I think it was in the Rifle magazine compendium of projects that there was a description of how to bore rifle chambers. I can't imagine doing it, my boring skills aren't up to the challenge, but might that be a possibility?