Beware These Two Mistakes

Beware These Two Mistakes | Shooting Sportsman Magazine
If point-of-impact testing shows your barrel(s) shooting too far left or right, your leads on crossing shots will be affected. Photo by Steve Oehlenschlager/

Judging from the mail I receive from readers and my general discussions with shotgunners, there are two areas of mistakes that many might not know they’re making.

Chamber Length

Most shotguns are sold with the chamber lengths clearly stamped on the barrels or barrel flats. But some shotguns were manufactured and sold with no stampings indicating chamber lengths. Others have unmarked aftermarket chamber changes. Purchasers of these guns are left in the dark as to the true lengths of the chambers. Many have resorted to “home methods” rather than chamber gauges to try to determine chamber length.

Why is this? First, shotgun chamber gauges are not commonly available. Usually they are in the possession of only gunsmiths and some gun dealers. But that’s not the only reason. A surprising number of shotgunners admit that they wouldn’t purchase a chamber gauge and would be unwilling to pay even a small charge to a gunsmith with chamber gauges to measure their chambers.

So I listen while they tell me a loaded 3" 12-gauge shell will fit in the chamber(s) of their 12-gauge shotgun and the gun will close fine. “Must therefore have three-inch chambers,” they falsely conclude. Sorry, not true. All unfired, fold-crimp 3" shotshells will fit into all 2¾" chambers of the same gauge and allow the gun to lock up, because in unfired, fold-crimp condition 3" shotshells in all gauges are only 2¾" long! Similarly, all 2¾" unfired, fold-crimp shells will fit into 2½" chambers and allow the gun to lock up fine. And so on. Worldwide, all shotshells are referred to by their length in fired condition. But all unfired, fold-crimp shotshells eat up about ¼" of their length forming the fold, or pie-crimp as it is sometimes called. So such “use-a-hull tests” can be completely misleading.

“Aha!” some of the hardcore, measure-for-free group comes back, “but I’ve also measured using a fired 3" shell.” Very sorry, but this is still an inaccurate and misleading procedure. If the 3" shell used originally was fold-crimped and the hull was made of plastic, there will be a certain amount of cone-shape crimp memory left at the front of the shell that will allow it to be pushed into the coned interior of the forcing cone ahead of the short chamber—allowing most guns to lock up.

The only accurate way to know chamber length is to measure it with a chamber gauge. Each gauge is made of metal to a specific width for a particular gauge and has sharply defined shoulders at the forward end to absolutely stop it at the front end of the chamber. A complete set—10 gauge through .410 bore—is available for $45 from Connecticut Shotgun Manufacturing Company.

Point-of-Impact Testing

I covered the correct procedure for point-of-impact (POI) testing in the September/October 2016 issue of SSM. But given the number of inquiries I keep receiving from readers on this subject, it merits review.

Point-of-impact testing is not pattern-testing. Pattern-testing is an attempt to quantify and measure the density and diameter of a shot cloud at a given distance from a given choke for a given load and shot size. In contrast point-of-impact testing needs no counting. It is solely an exercise to learn if the barrel(s) of a shotgun and/or its screw-in chokes (if any) project the shotstring abnormally high, low, left, right or a combination of these from a dead-on point of aim.

To accurately measure POI, the most important procedure is to be certain the test shotgun is not being held in a shooting position by the shooter. Got to get the human element—mis-mounting, flinching and shoulder shoving—out of there. To do so you have to rest the gun’s forend on a couple of shot bags on a benchrest table at a rifle range, or on a picnic table or at least on the hood of a vehicle so that all you’re doing is pulling the trigger and your shoulder is acting as a backstop. Then you need a simple three-inch-or-so-diameter blackened circle drawn on a minimum 3' x 3' sheet of paper or cardboard to aim at downrange. To be the most revealing, the POI firing distance should be 30 to 35 yards. Try to use a tight choke; it doesn’t matter what shot size or shot type. Fire about five shots carefully aimed at the blackened circle. Test one barrel/choke at a time, and then change the target after five shots.

The result will be a multiplicity of holes in your target. The greatest density of the resulting holes will tell you clearly where your shotgun barrel is shooting. If the center of the pellet registration is not more than 10 inches left or right of the blackened circle, great! If it is high, even as much as a half-pattern diameter, no problem. But if the POI is low, you will have a lifelong hitting problem, as most bird hunting shots are level or rising targets. And a too-far-left-or-right POI will affect leads on crossing shots.

If you ever take the time (and every serious shotgunner should) to test your shotgun barrel(s) for POI, you will be surprised how common off-shooting barrels and choke systems are. And in my experience, having tested hundreds of shotguns, this occurs throughout all price points of shotguns. So beware.

If your barrels do shoot off, you’ll have to take that up with the manufacturer. Don’t be surprised if you don’t get a lot of sympathy about correcting it. And just what you want to do with such a shotgun later will be left up to you. Sometimes stockfitting or eccentric choke tubes can correct the problem. But with fixed-choked shotguns, you usually also will need careful, corrective choke reaming.

To consult with Tom Roster or to order his new Advanced Lead & Bismuth Shot Handloading Manual, his HEVI-Shot and HW 13 reloading manual, or his instructional shooting DVD, contact Tom Roster, 1190 Lynnewood Blvd., Klamath Falls, OR 97601; 541-884-2974,

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