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Turning the OD is way easier to do accurately than an inside ream.

Have a wee go with one piece of brass and measure it up once you're done.

Just the faintest skim off the outside that removes...

Have you considered using a sizing button or expander mandrel to push the "donut" to the outside and "skimming" it off with a neck turn

Initiating the process would at least show you if you indeed...