Good question, as most shotshells have been lengthened far beyond their original design parameters to allow more room for componentry. The patterning board proves that this "more is more" school is not always the best, or even a reasonable approach.

If we accept that every shotgun has two forcing cones, one at the breech and one at the muzzle (the choke), shooting a 2-3/4 inch shell in a 3-1/2 inch chamber is just asking for problems, and I've seen them. For example, the accoladed Browning Citori XS, XT, and Citori 525 clays guns all have 2-3/4 inch chambers. So do the Sporting Clays Browning Gold versions, and the cost of separate barrel inventories for these models strongly suggests that it is not by happenstance.

When hot gas pushes a wad down the bore, the shot mass wants to expand. Giving a smooth transition from hull to bore with minimal shot deformation and upset is the primary requirement of good forcing cone design. If we fire a 2-3/4 inch hull in a 3-1/2 inch chamber, what happens during the unsupported "jump" is obvious: our shot mass wants to migrate out to hull outside diameter size. It tries, and then is immediately reconstricted, and overly constricted, when the forcing cone ramps the mass down to the inside diameter of the barrel.

If you measure the outside diameter of a 12 gauge hull you'll find it to be around .800 inch. Our chamber, which must be a "slip fit" for the hull, is necessarily larger than that. Starting out at over .800 in., we expect our wad to seal at that unsupported level, and all the way down to a 12 gauge bore diameter which varies from .729" (American designation) or .725" diameter (European designation).

A .075 jump is a lot to ask of a plastic wad, and some gas blow-by is to be expected. Shooting 2-3/4 inch shells in a 3-1/2 inch chamber is opening a can of worms, and just asking for poor pattern efficiency.

There is a lot to be said for shooting the shell length that matches your chamber length. The same effect is also present in shooting 2-3/4 inch shells in a 3 inch chamber, but with only one third the gap, is far less pronounced and problematic. I've shot outstanding patterns from 2-3/4 shells out of 3 inch chambers, and many hunters either don't care or feel that the potential reduction in pattern efficiency is not significant compared to the added flexibility a 3 inch chambered field gun offers them.

Most manufacturers agree, as the majority of field guns sold today in 12 or 20 gauge are fitted with 3 inch chambers. There are exceptions, though, and they include almost all competition guns (trap, skeet and sporting clays models). Again I'll cite Browning Arms. The Citori O/U Superlight Feather, clearly a dedicated upland field gun, comes with 2-3/4" chambers; so do most "London best" side by side field guns made today. In years past, many 12 gauge London guns were built with 2 inch or 2-1/2 inch chambers. The short shells and light loads simply pattern better than heavy "over square" loads.

Only the patterning board reveals it, but unless you think you need a steady diet of 3-1/2 shells for something (and I sure don't), it is wise to steer clear of 3-1/2 chambers. That is one of the reasons all my 3-1/2 inch chambered shotguns have found homes elsewhere.

Advances in shot, such as the popular Hevi-Shot, have in large measure negated the need for the 3-1/2 chamber, certainly when hitting something as fragile as a turkey head. For long range pass shooting at geese they might have a place, but you will likely pay a performance price with that same gun on the dove field. Like the re-rise and demise of the 10 gauge due to the early, comparatively poor steel shot loads, the sun is setting on the 3-1/2 chamber in 12 gauge for general use.

Three inch chambers remain a reasonable compromise in 12 and 20, and are not available in 16 or 28 gauge. Yet another reason, I believe, that both gauges generally pattern better than they are "supposed" to.