doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Barrel convergence results

After hunting yesterday, I attempted a barrel convergence test to see if my barrels shot to POA by firing left and right barrel at the NRA target from 20 yds. I used RST 12 ga 2 1/2� #6 lite copper shells. The first target is my more open choked box lock and the second my Purdey. I was fairly pleased, but I leave it to the experts here to tell me if I have a problem. My pattern was a little low on the boxlock test.

The regulation and POI looks fine to me, Owen. I see nothing to be concerned about. If the box lock shoots both barrels a tad low next time you check it, there are easy ways to remedy that. I never make adjustments on the basis of one patterning outing.

Bad part is ............ now you have no excuse for missing!

SRH

P.S. The only thing I would do differently is to not shoot both barrels at the same paper. Shoot them on separate pattern papers. It is possible that they could be compensating for each other's flaws, when you shoot two patterns on top of each other. Though not very likely, it can happen.

Yes, you do have problems. Your papers are full of holes.

What is the optimum distance to check barrel convergence? Would 30 or 40 yards yield a more accurate check? I've shot patterns at close distances (20 yds. skeet) and found them to true up at 30+ yards or more. I'm speaking of left/right symmetry here.
Karl

When looking at barrel convergence, I like about 15 yards.

It's easy to find the center of the pattern at that distance.

Contrary to the belief of some of our members the average flight path of a shot cloud is, in fact, a straight line as far as lateral convergence goes. Vertically, all firearms are compensated for gravitational drop and a shotgun is no exception.

Patterns that are coincident at 15 yards can only be off by the distance between the muzzles at twice that. If a gun crossfires at 15 yards, expect double the error at 30.

Vertically, just like a rifle bullet the center of your pattern will cross the sight line twice. Once on the way up, and once on the way down. Thus, the vertical POI must change with distance.

You do have a problem
Send the gun to me and I will test it
and shoot it and do all I can to fix it.
You should get it back in a couple of years, maybe
Mike

For POI testing, barrel gurus, like the Eyster folks in Ohio, who are pretty good barrel men, recommend at least a 3-shot group, and 5 even better for each barrel to assess point of impact. Look where shot is most dense and that's POI. Reason being, it's pretty easy to flinch on one shot.

Thanks for the thoughts. Mike, sounds like I need your help. Gun is in the mail with a couple of flats of shells for testing. Send back whenever!

Actually I was trying to reproduce McKay Brown�s convergence test after reading about his methodology in the Dallas book. I found it most interesting that cast can alter the results of this test. I had cast added out to my 3/8ths for both guns so I was interested to see if this affected the results of the pattern. I�ll try some of these other ideas next time out.

It ain't broke.

Exactly. It would seem at first glance that this would make it hopeless to hit anything at 30 yds. or beyond, however, keep in mind that the pattern enlarges as the range increases, too. This is the only reason some guns will hit anything at longer distances, and also the reason some people who have never patterned their guns prefer open chokes. Big patterns make up for sorry workmanship in regulation, to a degree. But, it's a poor, poor tradeoff.

Some people are so handicapped by poorly regulated doubles that they would be amazed at how much their hit ratio would improve with a properly regulated gun. Same goes for people who shoot single barrel guns, but have never shot them to see if POA and POI are coincidental.

SRH