Doug, my reference was to 2 3/4" SHELLS, not HULLS. Your question was, why not shoot 3" 12ga SHELLS in 2 3/4" chambers. My answer was that since 2 3/4" SHELLS are readily available and inexpensive, the question should rather be . . . why do it in the first place? And factory 2 1/2" SHELLS are more expensive than even very good 2 3/4" target loads. But then you know that, or should.
Gough Thomas repeated the same experiment as Burrard, and came up with the same results: Factory 2 3/4" shells, loaded to appropriate pressure limits for the gun in question, produced no higher pressures when fired in a gun with 2 1/2" chambers than did 2 1/2" shells. (I believe he used factory Eleys.) It's what's IN the shell--see both Burrard's and Thomas' reference to American factory 2 3/4" loads, which have a higher service pressure than their Brit equivalents--that causes the problem, not the extra length of the hull.
And by the way, if you actually measure the fired length of some of those 67.5MM hulls which are OK for 2 1/2" guns, and compare them to some American factory 2 3/4" hulls, you will find that they are almost the same length. In some case, exactly the same length. And in a few cases, the 67.5's are longer. When this discussion arose a few years back (when I thought we'd finally driven a stake through its heart), I assembled quite a collection of hulls, put them side by side, and found those very interesting results. So, if those 67.5MM shells are all OK in all 2 1/2" guns (all properly inspected, Geno), then why would a 2 3/4" hull generating the same pressure (or less), which is in fact no longer (or maybe even shorter), be a bad thing?
I no longer have that nice collection of empty hulls. However, I recall quite clearly that I did not find A SINGLE ONE that actually measured 2 3/4" in length. They were all somewhat shorter. If you have a chamber that's 2 5/8" rather than 2 1/2", chances are quite good that most 2 3/4" hulls will not enter the forcing cone, and the rest will enter it only a very short distance.
I should add here that, as in all things, there are some exceptions. There are some 2 1/2" guns--usually pre-20th century, and with very short and abruptly tapered forcing cones, where even the 67.5MM shells (to which Geno just gave his blessing for all short-chambered guns) won't work. You can end up with blown ends, etc, and you have to use shells that really do measure 2 1/2" when fired, or less. But these cases are pretty rare. And the solution is pretty simple. If you fire a 67.5MM shell or a reloaded 2 3/4" shell in one of those guns and if you blow the end off it, stop. Don't do it any more. Stick with the real shorties.
And Doug, since you seem to be big on facts, not theory . . . do what I did. Assemble yourself a collection of fired factory ammo, in 2 1/2", 67.5MM, and 2 3/4" lengths. Then come back and tell us how 67.5MM shells--which can, in fact, measure as longer or longer than 2 3.4" hulls--are always good to go in 2 1/2" chambers. (And, by the way, I did not find ANY 67.5MM hulls that were as short as 2 1/2". Which means, by your previous statements, that these are NOT the proper ammunition, because they are longer than the chamber in question--assuming it's really 2 1/2".) I guess those 67.5MM shells must be good not because they're the right length, but because they're measured in MM. And since that comes from Europe, and the guns come from Europe . . . suppose that's one way to look at it.
Last edited by L. Brown; 07/09/10 07:44 AM.
