Control of "BREECH PRESSURE"is the key safety concern when shooting 2 3/4 shells in British 2 1/2 chambered guns.Quoting from Eley literature they state,"Some 12G,cartridges having 2 3/4cases can be safely used in guns having 2 1/2 chambers!"
If you measure your typical 2 3/4 shell you will find that the actual loaded length is 2 1/4 - 2 5/16. After firing the case length measures 2 5/8-2 3/4. [You will also find that cartridges sold as 2 1/2 have almost the same dimensions as the 2 3/4 shell!]The thickness of a plastic shell case is .020 approx; therefore this 1/4 inch extension of the shell case can easily be accomodated, without constriction below bore size, in the the forcing cone as it transitions from chamber diameter to bore size.
Noted English balistics experts, Burrard and Garwood [Gough Thomas] have published test results that show 2 3/4 shells correctly loaded for the design pressure of 2 1/2 chambered guns can safely be used in nitro proof [In Proof] British guns with 2 1/2 chambers.
I use the following reloads in 2 1/2 chambered guns:-
22.0 Gr.Dupont SR 7625 powder.
Winchester AA Hulls.[or similar 2 3/4 case length shell]
Remington 29930 wad.[or similar]
1 1/8 ozs; shot.
Approx breech pressure is 6900 LUP.

Roy Hebbes