Well before Bell ever ran his "Finding Out for Myself" tests--which provide still more confirmation of the practice--the late British gun gurus Maj Burrard and Gough Thomas both reported on the practice of shooting 2 3/4" shells in 2 1/2" chambers, which has been common in Great Britain for some time (provided that the shells in question were made within the pressure parameters appropriate for 2 1/2" guns). And both Burrard and Thomas concluded that the practice was perfectly safe--although in Burrard's case in particular, he can be misquoted on the subject if you do not read ALL of what he said. Thomas gives the actual pressure figures for a 10-shot string, 2 3/4" Eley Grand Prix shells fired in a 2 1/2" chamber; same in a 2 3/4" chamber. There is no difference in pressure and velocity from one to the other.

It is very easy to reload 2 3/4" 12ga hulls, especially 7/8 oz target loads, to pressures of under 6,000 psi. At least here in the States, most of the 2 1/2" shells we can find are field loads vs target--heavier shot charge, greater velocity, greater pressure. So chances are you're actually doing your old gun a favor by shooting low-pressure 2 3/4" reloads in it, unless you can find a good source of low-pressure factory 2 1/2" shells.