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

At what distance is a double gun (SxS or O/U)regulated for barrel convergence? If memory serves, it's about a hundred yards for a double rifle, but how about a smoothbore?

Mike

Mike,
The quickest thing to do to learn a bit about this is to grab a set of calpers and measure across the outside of some barrels at the muzzles and then back a defined number of inches and calculate the convergence to a value that you will use to compare other guns to. I did quite a bit of this when I converted a 20g to a 28g and was having convergence issues.

I didn't think there were set distances; convergence is about regulation; regulation is about dealing with muzzle jump, recoil and the "up and away" recoil continuum, muzzle velocity/barrel time etc. All of these variables are affected by the characteristics of the individual gun and load, and trial and error is the way to get there.

I guess there would have to be some ball park figure to start with though, and a quick and dirty rule of thumb for lower grades which will never be indulged with that time-consuming regulation process.
RG

Now this is straight off the top of my head, so do get too upset with me if I have overlooked something - but if you coud turn a couple of pieces of delrin or something simliar to match your choke and then center drill for two laser pointers - I'll bet you could get close to the phyiscal convergence. A good friend with a Lathe might be required.

The trouble with the above method would be if the muzzle end of the choke is not concentric - which could be your problem or solution.

Possibly on a $100k shotgun that process of regulation is used. The H&H video shows shooting a shotgun for regulation by their guy. I suppose that could be challenged as well, since it's not the buyer that is shooting it nor his loads.

Me? I think that virtually all "production" guns that are not made to a customer as a "best gun", are regulated by dimension and most are not shot by the factory to check regulation.

You're right, Chuck, that trial and error regulation by hand is reserved only for the highest grades (if they even still do it at all). I understand in days gone by the owner could have the opportunity to participate in regulation, and old labels inside case lids told the owner what load the gun had been regulated for - not just for DRs, but shotguns, too. In a 12 it was commonly 1 1/8oz of #6 over a charge of C&H No 6(?) or 40-something grains of schultze.

We still don't know what the quick and dirty rule of thumb is/was for the vast majority of guns! I imagine it's just as much art as science, though.
RG

The question arises largely due to poor shooting with my Grulla SxS last Fall chasing roosters in Montana. My host and friend, who has forgotten more about dbl. guns than I will ever know, suggested I do the "16 yd test." Pattern the gun at exactly 16 yds, and bend the stock 1/16" the direction you need to move the pattern for every inch the pattern is off.Thus a pattern two inches right of center would be 2/16 or 1/8 bend to the left. Doing this 16 yd test, the left barrel was 2" rt., and the right barrel was a whopping 5" right (center of pattern). Pattern was the same whether aimed like a rifle at the patterning board, or point and shoot. FWIW, I am right handed, left eye dominant, and use a dot on my shooting glasses. Supposedly the 16 yd. test is for gunfit only, not convergence. Also FWIW, my right eye aligns perfectly over the rib.

Drawings from the L C Smith "Plans & Specifications" book show a convergance of .3385" from Breech C/L to Muzzle C/L, for a 30" 12ga FW bbl (1.156" vs .8175"). C/L's thus converge at 72.452" from muzzle & are crossed by 15.453" @ 40yds. Included angle is thus 0° 38' 47". Top rib is .566" above C/L at breech & .355" at muzzle for an angle of 0°24' (rib dips below tops of bbls at muzzle). I suspect that in the lower grades in particular the vast majority of these guns were never "Regulated".
Here's the data, anyone built a set Yet.

I have read that the patterns should superimpose(shoot together for folks in Rio Linda) at 25 yards. I've also read 40 yards. Some people even think(probably not a good word) that the pattern centers can somehow be fairly far apart at the muzzle and miraculously come together at the magic 25 or 40 yard mark. To me that's pretty darn silly. The chamber centers are maybe 1-1/4" apart and muzzle centers are maybe 3/4-1" part. How far could the pattern centers go apart on their way out to the desired convergence point? I shoot any newly acquired double from a rest, on a bench with the aiming point at 10 yards. My latest old Lefever, full/full shot a pattern the size of a large coffee cup, with a shot from each barrel. If they don't converge at close range they darn sure won't converge at longer distances. I have not observed the often quoted myth(IMO) about barrels not shooting together from a steady rest but will when shot off-hand. IME, if they are properly regulated they will shoot together all the way out from muzzle to any place within shotgun range. There's no black magic or mystery here.