I shoot Chambermate 12/20 adapters in a Brit twist hammer gun with 2-1/2" chambers, works fine. Might be because the nose of the adapter slides into the 12ga forcing cone, not sure. I have been using mostly 2-1/2" 20ga black powder loads, but have used some regular 20ga 2-3/4 promo loads with no issues.
When I was looking for a way to shoot 20ga shells in an old 16ga SxS, Ivory Beads told me that I could cut down a set of the old GaugeMate Silvers to fit the short 16ga chambers, but I never got around to buying the GaugeMates.
ChamberMates and GaugeMates are both long gone, but sets turn up occasionally.
