Joe4d, I bought a new 686 pro like yours a few years ago. Shooting over a rest, and trying a variety of factory ammo, accuracy was dismal. I checked timing, B/C gap, end shake, bore, charge holes. Everything appeared to be as it should. Till I checked the barrel crown, if it could be called that. It was about he crudest I'd ever seen on a new gun. S&W recrowned the barrel, and afterwards the gun was as accurate as my other S&W revolvers.

I don't know how to accurately measure S&W groove diameter either. But as long as throats are not smaller than groove diameter, Id just shoot the gun unless accuracy is obviously poor......ymmv

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