Testing Point Of Impact


Dear Technoid:

Does it make any sense to use slugs to check for point of impact? Shooting a few slugs at a target is certainly quicker and easier than shooting a number of shotshells at pattern sheets then guestimating the center of the pattern. Your thoughts please.

Thanks,

John

Dear John,

Testing point of impact (POI) with a slug is a good theory. It certainly would make life a little bit easier and might actually encourage more people to do this essential bit of testing.

Unfortuntately, I don’t think it’s a good idea in most guns. Slugs tend to be vigorous shells and produce different barrel harmonics than most standard loads. Shotgun barrels tend to flex when they are shot. The heavier the load, the more they flex. The more they flex, the more POI changes.

Also, gunfit changes very slightly with the amount of recoil. Light loads are more controlled with the hands. Heavy loads drive the gun back more into the shoulder for different dynamics. In theory it shouldn’t matter as the shot should be out of the barrel, but in practice it does seem to affect where the shot goes.

Top barrels on O/Us have a tendency to shoot high. I know that when I test varying heavy and light loads, my top barrels often shoot higher with the heavy loads. SxS guns will throw the shot from the right barrel further right as the power of the shell is increased.

Because shotload and velocity can affect POI, I think that it is best to test POI using your standard working load and guesstimating the center of the pattern. After you do a few, you can get pretty accurate at finding the center. If your gun has screw chokes, it makes sense to test with the fullest choke you own. That will make reading the POI a bit easier, even at 40 yards.

I often test my POI by simply shooting at marks on a vertical dirt bank while someone looks over my shoulder. It isn’t super precise, but if you have enough choke in there, you can get a pretty good idea of what is happening. You’ll need an experienced observer and enough shots. Of course, it’s the lazy man’s way out… just fine for your slothful Technoid.

If you are shooting a double barreled gun, when you check POI also make sure to check barrel convergence (BC). You would think that getting a double to put shots from both barrels in the same place wouldn’t be too hard, but it is. It is also very dependent on the type of shell used and can vary to a noticeable amount when going from the lightest to heaviest load. BC is just like POI in this respect. If you can get your two shotgun barrels to print within 5″ of each other at 40 yards, most manufacturers feel that you are doing OK. One major American maker is very proud of their 6″ maximum permitted BC.

I’ve said it here before, but it bears repeating- if you shoot a particular gun well, it might be best not to do too much testing. I had a Belgian solid rib Superlight bird gun that I just shot marvelously. One day the Devil made me check it’s BC and I found that the top barrel shot impossibly high. I’d been killing tons of birds with that top barrel, but now that I knew it’s dirty little secret, I ended up selling it. The Superlight I replaced it with is technically perfect, but I don’t shoot it as well. Go figure.

Best regards,

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

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