Since you probably won't be able to buy 6.5 Ruger brass for a while (I can't find any on Midway yet) I would assume you are going to have to buy factory loads, shoot them and reload the expended brass. If that is correct, then what I would do would be to buy a box of factory loads and mic the outside neck diameter.
For example if the factory loads mic'ed .296" (and there will be some variance) then you will have some kind of idea what to ask the gunsmith to make the neck. In that case you could choose to avoid neck turning by asking for a .296" + .003" = .299" neck. That would allow you or someone else who wound up with the gun later to shoot factory. If you wanted to turn just enough to clean the necks up some then you could ask for a tighter neck.
I have a custom barreled 6.5 rem mag that has a .292" neck. I can buy Remington brass for that caliber off Midway or elsewhere and I have found that the new cases have a thick neck of between .015" to .017". So I have to turn the necks down to .0125" (.292" - .264" = .028" - .003" = .025" / 2 = .0125").
A friend liked my 6.5 rem mag and wanted me to help him in the decision making but he wanted the possibility of shooting factory ammo. So we bought a box of factory ammo and they measured .295" or less. His gun will have a .298" neck which is a good size cause I can neck turn the thicker factory cases just enough to clean them up (.298" - .264" = .034" -.003" = .031" / 2 = .0155") or he can shoot factory.
So start with the brass you are going to use, either by purchasing some if you can find them or buying a box of factory and that will help you make the decision.
Edit: I just went back and saw that you were forming brass from other calibers. In that case I would form a few and see what the neck thickness turns out to be. Not a bad idea to be able to shoot factory ammo just in case though.
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Last edited by woods; 03-25-2008 at 10:39 PM.