Quote:

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It's time for a chamber cast.
Find out exactly what is going on in your chambers before you fret over drawings.
What's real in your gun is more important than what appears on a spec sheet.
--tinker

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What he said. Even if you had to get the neck of the 8x64 opened up to the slightly larger diameter with a neck reamer after the rechambering, I can't see .005" being of any significance. But do get a chamber cast done, & find out exactly what you're dealing with rather than maybes.
Steve