Barrel Convergence


Bruce,

I have a Beretta 682 Sporting that I use for skeet. It fit me perfect right out the box, & as I recall, both barrels had good points of impact.

This past grouse season I purchased a Beretta Ultralight, & it didn’t feel right. It pointed high (saw too much rib) & to the left when I mounted it. I sanded the stock to the point where I can now see straight down the barrel & the rib appears flat. However, when I checked it out by shooting skeet, I did poorly. (I shoot low gun using the Orvis Method). I don’t think the gun is too whippy, as I have 28 inch barrels, & I occasional shoot my Buddy’s 28 Ga O/U with excellent results.

This past week, I checked the point of impact using full choke at 20 yards with light skeet loads. The bottom barrel was 6″ low, while the top was right on. Is this difference between top & bottom barrel bad enough to get Beretta to fix it? Is there a good reason for this difference?

Does muzzle jump occur fast enough to lift the point of impact? Should one expect to see more rib on a field grade gun? I suppose I should now repeat the above tests with my actual field loads. I reload 1 1/4 oz. of hard #7 shot for grouse. For pheasant, I buy premium shells with 1 1/2 oz of lead. The kick is noticeable. Because of the large muzzle jump, I can’t get off a second shot with my usual rhythm when necessary.

I would appreciate your help in correcting this problem. I have thought of shooting the top barrel first, but would have even more muzzle jump than before.

Thanks,
Glenn

Dear Glen,

Welcome to the world of a light gun. The lighter a gun is, the more perfectly is has to fit you in order to perform adequately. With a light gun (and especially with heavy shells), any little flaw in gun mount or stoppage in swing will be magnified. Of course, we have to use light guns in the field or our knuckles will be dragging on the ground. It’s all a compromise.

It sounds as though you have two issues:

1) muzzle jump on shooting, and
2) barrel convergence.

Barrel convergence first: The basic design of the O/U “wants” to shoot the top barrel higher than the bottom because the axis of the top barrel is higher. To counteract this, the manufacturers build the barrels closer together at the muzzle than at the breech so that they (in theory) converge at a certain distance. At least that is their intent. In fact, they usually are off by a bit and most O/Us shoot their top barrels higher than the bottoms. The heavier shell you use, the more you will notice this. Barrel convergence will vary slightly with the power of the shell.

Is 6″ separation at 20 yards too much? I think so. A top barrel which shoots 6″ higher than the bottom barrel at 40 yards would be right on the edge of acceptability. That 6″ at 20 yards will grow to 12″ at 40. That’s too much when you figure that your killing pattern on pheasant from a full choke at 40 yards isn’t over 18″.

To do a proper barrel convergence test, use the same full screw choke in each barrel and test it with your hunting load. If you feel that you will shoot as far as 40 yards, then test at that distance. Test by aiming the shotgun like a rifle, but do it standing up or the recoil will kill you. Use an aiming point on your piece of paper and take three or four shots from the same barrel/choke/shell on the same piece of paper, so that you get a good pattern overlay and can really tell where it is striking. Then do the other barrel. Read your patterns from the back of the paper and the holes will be easier to see. But remember, that your are judging point of impact (where the center of the shot cloud is) rather than any kind of pattern percentage. That’s something entirely different.

The problem with poor barrel convergence is that no matter how much you adjust the stock to make the gun shoot to a different point of impact, those two barrels are never going to “agree”. When one is on, the other is off.

There are three ways of dealing with a barrel convergence problem, none of them good.

1) use an eccentric choke available on a custom basis from Briley Manufacturing, 1230 Lumpkin, Houston, TX 77043, tel: 800-331-5718, . It works fine with the only caveat that you have to always index the screw choke perfectly and make sure that you only use the biased screw choke in the barrel for which it was built. In solid choke guns you can just cut the choke on a bias. I had that done on a Parker Repro and it worked fine.

2) pull the barrels apart and resolder them correctly. Expensive.

3) sell the sucker.

Muzzle rise on firing is an entirely different situation. This is exacerbated by a light gun and/or a heavy shell. This is one of the compromises you make in any field gun. If you are really desperate, you might try porting the barrels. Normally, I don’t recommend portng because it is usually associated with target guns. I don’t feel that it makes very much difference in a target weight (heavy) clay target gun using target (light) loads. In a light field gun with heavy field loads, it might help very slightly. Strange that the porting market is aimed at the target shooter where it does the least good. The big field loads that some people use might actually generate enough gas at the ports to do some good, compared to the relatively puny target loads. The reason that porting and muzzle breaks work so well on rifles and pistols and not so well on target shotguns is directly attributable to the difference in the amount of gas generated and the pressures at the ports. Port pressure makes porting work. More pressure equals more push.

I think that a better approach to controlling muzzle rise might be to increase the amount of pitch you are using. Pitch is the angle of the butt plate vis a vis the rib. If you insert a shim between the TOP of your butt pad/plate and the wood, that will increase pitch and might help keep the muzzle down. Worth a try. Your gun probably comes with around 2″ of pitch, so increasing it to 4″ might be a good experiment. You can always take the shim back out. A shim the thickness of two quarters might do it. Remember, just shim the top of the pad in order to increase pitch.

If you do try to send the barrels back to get the convergence problem fixed, I would be very interested in hearing what Beretta’s service department response is. Be prepared to wait, that’s for sure.

Best regards,

Bruce Buck
The Technoid writing for Shotgun Report, LLC
(Often in error, never in doubt.)

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