Re: Shiny New 338 Lapua Brass, Now What?
Sounds like you have the sizing idea down, which is just to run the brass through each of the two sizers (body and neck) that you have.
As far as what kind of neck GAP's reamer has, I have no idea. George or one of his 338 customers should be able to tell you.
If you seat a primer without using the Sinclair primer pocket tool, how deep is it compared with the case head? If it is already seated a bit below the case head, I would be slightly wary about removing too much more material from the pockets, although I don't know of any particular harm that it would do. Is Sinclair's tool adjustable? If not then its depth is probably carefully set by Sinclair for large rifle primers and it would be hard to hurt brass by cutting too deep with it. Those are two sides to the argument as I see it, sorry I can't give you anything more absolute.
Have fun and good shooting,
Empirical results rule!