Unfortunately there are few choices in 264 Win Mag for brass. Don't ask why I'm shooting a 264 and not something else more popular. I deburr, trim, ream, and turn my brass and have had a problem with Winchester brass. In fact, I favor it over Remington brass and some others. Ok.
Before thinking of altering the die, I decided to look at my RCBS shell holders, I had three of them. Measuring them, I found 0.005" variation in length from where the brass head resides and where the ram bottoms, and coincidentally the one I'd been using would result in the longest head space.
I switched to the "taller" of the three shell holders and sized through my Redding body die, which DOES size the case body and shoulder. It does not size the neck (as bushing sizes that). Tried the sized case and chambers effortlessly.
Verdict - Be aware that RCBS standard shell holders have length variation high enough to cause head space trouble in belted magnums.
Case closed!
There alot easier way to do this, you still going to have chamber problem when you get to 4 to 5 firings
