If I understand your term "thinness of the joint between the barrels at the breech" , then changing gauges would increase that measurement.
Roughly the increases would be .060" increase for a 16 bore, and .110" for a 20 bore. I guess I don't understand why one of those might not work.

Last edited by Daryl Hallquist; 05/08/22 10:51 AM.