Smallbore, did you discuss reloaded 2 3/4" cases with the proofmaster, or did you refer only to factory 2 3/4" shells? Many of us here have clearly drawn that distinction--as did both Burrard and Thomas, with their very clear warnings NOT to use American factory 2 3/4" shells in guns with 2 1/2" chambers. All of us agree 100% with those warnings.
We also have the issue of the nominal "67MM" Gamebore shell--which, you will note, I measured with a digital caliper to have a fired length of 2.63". That's almost exactly 2 5/8", or 1/8" longer than the chamber length of a 2 1/2" gun. Yet right on the box, Gamebore will tell you that that shell is suitable for use in guns with 2 1/2" chambers. If case length is the problem, how can that be so? Answer: case length is NOT the problem, per Burrard, Thomas, and Bell. IF the case is loaded to pressure parameters appropriate for the gun in question--and most of us on this side of the pond are reloading VERY low pressure 7/8 oz reloads for our short-chambered 12's, around 6,000 psi--it doesn't make any difference whether the case is 1/8" or 2/10" longer than the chamber. Tests by Burrard, Thomas, and Bell--using instruments to measure pressure--have shown either no increase in pressure from the longer case, or only a very slight increase in pressure.
When the length of the fired hull exceeds the length of the chamber by no more than 1/4", the danger comes not from the extra length, but from what's inside the hull to start with.
