Doug, if the mouth of the shell extends into the BORE--that is, PAST the forcing cone (note that when you measure BORE diameter, you do NOT do it in the cone!)-- that would indeed be a potential problem. But we all know equally well, don't we, that it is highly unlikely for that to happen. Unless, of course, you do something like maybe shoot a 3 1/2" shell in a gun with 2 1/2" chambers. (If you could close it on one of those.) Or run across a gun with extremely short forcing cones.
As for the part about the short and sharply angled forcing cone . . . darn Doug, you must have a lot of time on your hands. If you'd simply read this thread, you would note that I pointed out that SOME guns--likely very old ones (pre-1900)--shouldn't even be fired with the 67MM/67.5MM shells, which the Brits and other CIP nations sell for use in 2 1/2" guns all the time. With the blessing of their proofhouses, I might add. Charles Fergus, in a long ago issue of Shooting Sportsman, reported that problem with 67MM shells in an 1880's or 1890's Brit gun he had, while the same shells functioned just fine in a 2 1/2" Brit gun from the 30's. And true factory 2 1/2" shells functioned just fine in his older gun. I'm not going to bother chasing down my ancient post, but I expect I might even have made reference to Fergus' report, therein.
That being said, the fact remains that the vast majority of 2 1/2" guns will work just fine with 2 3/4" hulls (none of which actually measure 2 3/4" when fired, and at least some of which measure no more than 2 5/8") loaded to appropriate low pressures. Were that not the case, the dozens of us on this BB who have been following that practice for a very long time would have reported problems, which you would have seen discussed here in great detail. But the fact is, it's a pretty rare 2 1/2" gun in which a fired hull will extend PAST the forcing cone and into the bore. In most cases, they'll barely reach the cone. And per data provided by an American back in the 30's, and by Burrard and Thomas, and more recently by Bell in his "finding out for myself" series, we know that that practice results in, at most, only mild increases in pressure--if we're talking a shell no more than 1/4" longer than the chamber of the gun in question. Most guns having a forcing cone of at least 1/4".
