I’ve talked vaguely about the desirability of proper barrel convergence before, but now it’s time to get into the gruesome details. Barrel convergence in a shotgun is simply the ability of a two-barrel gun to shoot each barrel to the same place. It’s not where the shotgun’s point of impact is. That’s different and is more a function of stock fit and eye alignment. Barrel convergence is the purely mechanical alignment of the two tubes.
With modern machinery, laser alignment and that sort of thing, you would think that getting each barrel lined up right at the factory wouldn’t be such a big deal. But it is. An over/under “wants” to shoot the top barrel higher than the bottom barrel. I don’t know which barrel wants to go which way in a side-by-side, but they definitely want to part company. Some of that is due to angle of recoil, but most of it can be blamed on the fact that shotguns with two barrels are wider at the breech than at the muzzle. Just look at both ends and it will be immediately apparent. They don’t shoot straight ahead. They angle in toward each other.
Twin shotgun barrels should be set up sort of like the machine guns on the WWII Spitfire and Mustang. They didn’t shoot straight ahead. The machine guns on each wing were set up to converge at a certain point in front of the fighter. Same with a shotgun.
Testing barrel convergence is easy. But first a caveat. If you have a gun that you shoot well and have faith in, don’t test it! Leave well enough alone. It’s like going to the doctor when you are feeling great. You can only break even or lose.
But if the gun is an unknown or is new and under guarantee, definitely test for convergence. Always test a new gun or one you are thinking of buying. To do so, first pick your distance. For a field gun, I like 20 or 25 yards. For a clays gun, 30 or 35, but you may have different preferences.
Paint your pattern plate or set up your paper at your distance. Mark a bull’s-eye in the center. If your gun has screw chokes, install the tightest choke in the barrel you will shoot first so that the pattern is dense and easily seen. Now aim the shotgun like a rifle. Don’t shoot it like a shotgun. We are testing the gun here, not your shooting skills. Fire three shots from the same barrel, one over the top of the other. This will produce a dense, easily definable pattern. Carefully note where the center of the pattern is in relation to the bull’s-eye aiming point. Now repaint or repaper, mark a new bull’s-eye, transfer the same tight choke to the other barrel if appropriate, and do the three shot thing again.
By noting the different striking points of the two barrels, you can easily discern barrel convergence or the lack of it. Ideally, the patterns should exactly overlap. But they probably won’t. Separated striking points can be caused by misaligned barrels or, to a slight extent, the recoil of the shells chosen. An inch or two or maybe three of separation at your chosen distance probably isn’t enough to worry about. Shotgunning isn’t that precise. But much more than that is going to be an issue. A typical 12-gauge pattern really has only about a 24-inch killing circle, more or less depending. How much of that do you want to give up?
I’ve seen barrel convergence so far off that the patterns almost wouldn’t touch. It’s not common, but it happens. With the best of brands too. One major maker told me that their standard of acceptance was 8 inches separation at 40 yards. Would you accept that? I sure wouldn’t.
If you do have a convergence problem, there may be solutions. If it is a new gun, send it back with the testing information and let the factory worry about it. If not, if the gun has fixed chokes, a skilled barrel man can sometimes grind a little bit here and there off of one side of the offending barrel’s choke and straighten out things remarkably. I had a 28-gauge Parker Reproduction that shot the left barrel far left, and it was brought back into perfect alignment by a little skilled honing. I lost a little choke constriction, but convergence was made correct. A screw-choke gun might receive help from Briley, which will make eccentric chokes for a gun. This may solve the problem.
So that’s the deal on barrel convergence. Boots off and in perfect alignment. Beer open.