JDW, there shouldn't be any problems shooting a factory 3" shell in a modern American double with 2 3/4" chambers. Why not? Well, because the SAAMI pressure standards are THE SAME (11,500 psi) for both 2 3/4" and 3" shells. However, why would one want to do so?

2 3/4" in a 2 1/2" chamber, on the other hand . . . For one thing, here in the States, 2 3/4" is our standard case length. All sorts of very low pressure reloading data is available for 2 3/4" 12ga hulls--lower pressure, in fact, than the vast majority of the factory 2 1/2" shells available in this country. Not to mention a lot cheaper! So why not use the reloads, to the appropriate pressure standards?

As far as the actual chamber length of a gun with a "nominal" 2 1/2" chamber . . . I have a pair of such guns, marked 2 1/2", and unaltered. Using my chamber gauge, I get just slightly over 2 1/2"--probably less than 2 9/16", and certainly less than 2 5/8".

Some measurements of once-fired American and British shotgun shells, all plastic, all crimp-closed (with a digital caliper):
Remington STS: 2.69"
Winchester AA: 2.70"
Federal Gold Medal: 2.71"
Gamebore Super Game (Nominal 67MM): 2.63"
Eley Blue (Nominal 70MM): 2.59".
Note that the 67MM Gamebore (marked on the box as suitable for guns with 65MM or 2 1/2" chambers) is only .07" shorter, on average, than American factory 2 3/4" hulls. That's almost exactly 1/16th inch--not likely to go very far into the forcing cone.

As for Gough Thomas, I refer once more to his "Gun Book", chapter entitled "Danger in Case-Length"--the conclusion of which is that there isn't any danger in case length. He specifies that the test he conducted used crimp-closed shells in 2 3/4" cases (to pressure standards appropriate for 2 1/2" guns) and had them fired in pressure barrels with, first, 2 1/2" chamber; and then 2 3/4" chamber. NO DIFFERENCE IN PRESSURE OR VELOCITY.

I'm not interested in convincing anyone here to use 2 3/4" hulls (even if loaded to low pressures) in 2 1/2" chambers, unless they wish to do so. I am presenting the evidence, which I find to be overwhelmingly convincing--and from 3 separate sources who conducted separate tests (Burrard, Thomas, and Bell)--that, providing the shell is loaded to the service pressure appropriate for a 2 1/2" gun, the 2 3/4" hull, in and of itself, presents no additional danger.

And Joe, that's a very interesting Parker hang tag. At the time that gun was built (1902), there would have been no difference in pressure between a 2 3/4" and 2 5/8" American factory shell. It was not until the 1920's that the higher pressure, "modern" American 2 3/4" loads appeared. Once more, the extra length would not have made a difference.