Bob;
I have 12 gauge Lefevers weighing from 6 1/2 to 8 lb on three different firing pin spacing's & of coarse different sizes of the breech balls. All have the same basic frame bar size. This would seem to me to be based totally on the weight/thickness of the barrels & have nothing at all to do with their gauge as they are all same gauge.

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