I note that essentially every 12 gauge shotgun made today, regardless of its designed purpose, is chambered for 3-inch magnum shells. The fellas at the trap club tell me it’s because 3-inch barrels pattern better even with trap loads. Something about the cone? Is this fact or fiction?
First of all, in my experience, you should believe about 50 percent of what gun club experts tell you. Some are genuinely knowledgeable. Others are not, but it doesn’t stop them from offering advice. The tricky part is figuring out which ones to listen to and which to ignore. Tread carefully.
As to your question, first, many target guns do have 2 ¾ inch chambers. My Browning break-actions do, as does my 1100. Some high-end target guns like the Beretta DT-11 have 2 3/4 inch chambers while some Krieghoffs have 3-inch chambers. And, many target guns from Beretta, Benelli, Fabarm and guns like the Browning Maxus have 3-inch chambers. So there are some of each. I have to think in some cases it’s a matter of manufacturing efficiency for a gunmaker to turn out 3-inch barrels for everything rather than make runs of 2 ¾- and 3-inch barrels.
A shotgun has a chamber, which, in a 12 gauge, is around .800”. The chamber is long enough to accommodate the fired shell. The next section is the forcing cone, which is the part of the barrel where it tapers from chamber diameter down to bore diameter. The cone can be under an inch long or rather longer. A longer forcing cone eases the transition of shot from the chamber to the bore, leading to fewer deformed pellets and therefore, tighter patterns.
The guys at the gun club could be wrong: theoretically, a 2 ¾ shell should perform best when fired from a 2 ¾ inch chamber because when it makes the jump into that last quarter inch of oversized chamber, bad things can happen ballistically: gas can blow around the wad; the petals of the wad can start opening, allowing the shot charge to spread slightly before it is crammed down into the forcing cone and into the bore.
Or, they could be right: if indeed you are losing velocity as gas blows around the skirt of the wad in the overlong chamber, you might get tighter patterns as a result, since lower velocities usually result in tighter patterns.
In practice, I don’t think you’ll find a huge difference in patterning between a 2 ¾-inch shell in a 2 ¾-inch chamber or a 3-inch chamber. Or you may. That all depends on the individual gun and the ammo you choose for it.
Since the only way to test this theory is to pattern a 2 ¾-inch shell in a 2 ¾-inch gun, then have that gun’s chamber lengthened and pattern it again with the same ammunition. No one is likely to do that, so you can fling around all the theories you want and there’s no harm in that. If you believe your gun is tight-shooting and hard hitting, you’ll probably shoot better whether the gun really shoots better or not.