I believe that it was Charles Fergus, in an article in Shooting Sportsman, who reported problems using even standard British 67MM shells in a British 2 1/2" gun from the 1890's. Some guns from that era had very short and sharply tapered forcing cones. Fergus reported increased recoil and the ends blown off the hulls. Only true 2 1/2" shells should be used in those guns. He fired the same shells in a gun from the 1930's with no issues.
I haven't fired as many reloaded 2 3/4" hulls in 2 1/2" guns as Joe has. But I shot a bunch of home rolled pheasant loads (1 1/8 oz) in a pair of Webley & Scott made Army & Navy doubles from the 30's. No ill effects . . . other than on the pheasants. Like Joe, I kept my pressures in the sub-8,000 psi range.
