I recently sold a rifle with a counter-bored barrel. I wanted to give prospective buyers some idea of the MW at the new muzzle. To do this:
1. I measured the distance from the original muzzle to the end of the counter-bored section using the male portion of a set of calipers.
2. I used my USGI TE gauge determine the distance from the old muzzle to where the gauge stopped in the new muzzle, and then subtracted the length of the counter-bored section to find the point on the TE gauge where it stopped in the new muzzle.
3. Then I inserted the TE gauge in a known muzzle and marked where the gauge stopped in the known muzzle.
4. By comparing the reading/mark on the TE gauge for the known muzzle with the reading on my MW gauge for that same muzzle, I was able to determine the MW of the counter-bored barre at the new muzzle.

This is not difficult, but careful measuring is required.
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