I bought a chino made ball-mic (locally) and with a tiny bit of adjustment with the tool that came with it, does a good job. Much better than stuffing about digital calipers for a radial thickness.
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Thanks for all the feedback, really interesting.
I will try my .291 bushing and if I experience any accuracy issues I will try reducing the tension with a larger bushing.
My experience is somewhat limited and relied on the support of a local gunsmith/BR shooter to help me.
I have just ordered a forster press and mitutoyo calipers so looking forward to making this a staple evening activity.