If I am unsure of headspacing, I routinely expand new brass to the next larger neck size, then size it down in the full length resizer so it is snug but not real tight before loading it. I had a 338-06 AI but am not convinced it added much if any velocity. Feeding could be a problem, and that is why shoulders were gentle until Ackley came along and sought a way for a bit more and some fame. Most of the improvements out to be from running higher pressures. In 1957, Elmer wrote that his .333 OKH should be updated with .338 since the advent of the 338 win mag would result in much better 338 stuff vs. the old 333 Kynoch which hadn't performed very well for him. If I wanted a bit more velocity, I would just use a 26" barrel vs. the customary 22-24".
