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[Kurt]

I had a wide base shooting the .45-90 cases in my Shiloh for several years made the base at .504". Shiloh normally uses a .505" base diameter so I made a base swage die to take the base down to .499"
My tight PP .45-90 I made the reamer with a .502" base to match the Starline brass closer which runs close to .500". This keeps the base ahead of the rim about .060" stretching the web from expanding larger.

Don I can see why your brass shot in the CPA .45-90 wont fit. When I build my CPA .45-90 I had a reamer made because I did not want that large base diameter they use at .5084" for the first .100" then going to .5072" plus the large .481" case mouth.