HOW TO Measure Odd Number Groove Diameter
The following question was posted on the old "ASK EACH OTHER" forum by ccrider on 09/20/2009:
"Can some one explain how you measure a slug from a 5 grove barrel so as to determine bore size?"
It was answered by Bob S Posted - 09/20/2009:
"Wrap the slug with thin shim stock; mike the shim stock first, it may not be "as advertised". Wrap the slug tightly,
and mike the resulting wrapped slug very gently, so you don't compress the shim stock into a groove in the slug
(which is an imprint of a land in the bore). Subtract twice the thickness of the shim stock, and that is the groove diameter."