Control of breech pressure is the key safety related issue when shooting british 2 1/2 chambered guns.Quoting from Eley literature they state,"Some 12 G, cartridges having 2 3/4 cases can be safely used in guns having 2 1/2 chambers"!
If you measure your typical 2 3/4 shell having a crimp closure, you will find that actual size is 2 1/4 - 2 5/16. After firing the case measures 2 5/8-2 3/4. The thickness of a plastic shell cases is approx; .020 therefore this 1/4inch extension of the case can easily be accomodated in the forcing cone as it transitions from chamber diameter to bore diameter.
Noted English writers and researchers, Burrard and Garwood [Gough Thomas]have published test results that show 2 3/4 cases correctly loaded for pressure, can safely be used in nitro proof ["in proof"]english guns having 2 1/2 chambers.
I use the following reloads:
22.0 Gr Dupont SR7625 powder.
Winchester AA hulls.[Or similar]
Remington 29930 wad [ Or similar]
1 1/8 oz;shot.
Approx; breech pressure is 6200 LUP.
I have fired in excess of 15000 rounds using this load in 2 1/2 chambered guns without any problem.
