Barrel Convergence Woes


O Exalted One,

May I be allowed to ask a humble question? I’ve been shooting a 682 gold E, 32″ for the past five months, I like it a lot, I shoot it fairly well 80-85 out of 100, very light, exceptional balance… I patterned it from a rest, and discovered the bottom barrel shoots 6″ low and 2″ to the right, the top 2″ low at 30yds, MOD choke, same choke in both barrels. To me that’s not good, I’ve been told that’s in the acceptable factory range. What do you recommend O Lord Of The Scatter Gun?

Philip

Dear Philip,

Let’s get terms defined first. Barrel convergence is how closely two barrels shoot together. Point of impact is where those two barrels shoot once you getting them both shooting to the same place. You have a barrel convergence issue. You want to get both the barrels to shoot to the same place. Then you can move that point of impact place to where you want it by adjusting the stock. Convergence is usually a choke issue. Point of Impact is generally, but not always, a stock adjustment issue.

So, we’ll discuss convergence. First of all, try a Full choke to get an even denser and more reliable pattern placement and an even clearer view of the center of your pattern, just to make sure. Additionally, using a different choke will eliminate a single crooked choke as the culprit. Also, try a couple of different shells, varying the speed. Use some light loads and some heavy loads. They will make the barrels resonate differently and may affect pattern placement. Also try shooting off hand as well as from a rest. That too can affect individual point of impact and barrel convergence.

If everything stays the way it is, you are still pretty much within most factory tolerances. Top barrels almost always shoot higher than bottom barrels unless you are dealing with hand-regulated guns. A 6″ to 8″ convergence separation at 40 yards is considered common and within the standards of most mass produced guns.

About the best killing pattern you can hope for with 1-1/8 oz of #8s at 40 yards is a 20″ circle if you can run a 77% pattern and use an 80% chance of a two pellet strike on a 6 sq/in target as your criterion for acceptable fringe. How much barrel mis-convergence you can accept simply depends on how much of that 20″ you want to give away. Dems da facts. There’s no nice way to put it.

While I would be very reluctant to give away 8″ of that 20″ to improper convergence, plenty of people do and most of them shoot better than I do.

Your options are:

1) redo your testing to find the shells that give the best convergence and then ignore the convergence issue;

2) get someone like Seminole, Briley, Eyster or others to recut or change your screw chokes to eccentric chokes to improve convergence,

3) sell the gun and hope you’ll luck out and get a better one- that’s where buying used and testing before accepting the gun makes sense.

As an aside, fixing improper barrel convergence is much easier with fixed chokes. You just have your guy grind a bit, shoot a bit, grind a bit, etc until they come out spot on with the load you want to use. Of course, you lose some choke in the process, but nothing is free in life. I had a 28 ga Parker Repro that shot it’s left bbl almost a full pattern left. It was moved back to perfect convergence by a little eccentric grinding. I lost some choke, but convergence was perfect.

Screw chokes are harder as you are already committed to the threading. Rethreading at an angle is more of a deal and requires a good amount of barrel wall thickness. I do know that Briley can screw choke a fixed choke gun by threading in the choke at an angle to fix misconvergence. This will allow you to use conventional screw chokes in both barrels, rather than rely on eccentric chokes dedicated to a single barrel.

As a general rule, when people ask me about barrel convergence my advice is the if you have a gun that you shoot well, don’t stir up the devil and test for it. Just leave it alone and have faith. If you have a gun that you don’t shoot well, especially one barrel, then test. Definitely test if you are buying a used gun. That’s what that three day inspection period is for.

Bottom line? Dunno. It’s really up to you. Do some extra tests as I described and see if things can be made to improve. If not, you either live with it, fix it with eccentric chokes or sell the gun. In various situations, I’ve done all three. There is no clear cut answer to every situation.

Best regards,

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

Advertisements
This entry was posted in Shotguns and tagged . Bookmark the permalink.

Leave a Comment

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s