I'm assuming from your post that there's no 2 3/4" (or 70MM) mark at all on the barrel flats, which would indicate the chambers were opened at the time of its modern, 3 ton reproof.
If that is the case, take it to someone with expertise in English guns. Have them measure barrel wall thickness, and in particular have them measure thickness immediately in front of the lengthened chamber. I seem to recall that the figure I've seen for minimum thickness at that point is .100"; elsewhere on the barrels, I'd hope for a minimum of .025" or better--but let the gunsmith provide his expert advice.
If you shoot the gun at targets and reload, there are all kinds of 12ga formulas that will get you down in the 5-7,000 psi range, using standard 2 3/4" hulls, which is plenty safe with that gun, assuming the barrels are OK. RST's MaxiLites are also very nice, light 1 oz loads for targets and light game. For heavier hunting purposes, both Kent Gamebore and Eley make some very nice 1 1/16 oz field loads. In spite of the fact that your gun has 2 3/4" chambers, you should avoid all American factory 2 3/4" shells. The pressure will be excessive for a gun proofed at 3 tons.
