My understanding of the gaugemate was that it is approximately shell length & has no forcing cone. You will thus on exit have whatever the dia of the internal hull measures, which will be slightly larger than the .729" 12ga bore. The charge will not of course exit directly into a .775" bore, but rather into the forcing cone of the 10ga which will be somewhat larger. I do not see any point where you would have a comparsion of a .752" dia to the .775" bore.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra