Mike, I'd be very skeptical that hull length is the problem, having fired thousands of 2 3/4" hulls in 2 1/2" guns. Some of the Brit shells marked 67 or 67.5 mm are in fact as long as American 2 3/4" hulls (talking fired length). Those Featherweights are supposed to be very low pressure, and they're certainly low recoil and low velocity. Maybe thickness of the brass head?
If you make Flatwater next year, remind me in advance and I'll bring some low pressure Federal 12's in 2 3/4" hulls that they made up for Midway. It'd be interesting to let some of the guys try them in different guns with short chambers and see if there are any issues similar to what you had. I shot those Feds in my 2 1/2" A&N pair without any problems, but have since parted with those guns and currently don't own a short-chambered 12ga.
