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

I thought that bowing in and then going straight for the last part of the assembly of barrels was called "swamping". It looked to be common before 1900 on many Parkers I have looked at.

bill

Bill,

That is the exact term the 'smith used, "swamped". This may explain why some cut barrels still meet or are too close to call.

Chief

To take this question a bit further, what happens to the placement of your patterns if say a 28 inch set of barrels is cut to 26, and they no longer touch? Does that mean the patterns will be that quarter inch apart at 40 yds? Sorry if this question is dumb, I just have a chance at a nice little sideby that was cut back when.
thanks
Rick

The point of impact won't change much if at all when you cut barrels unless there was something really radical in the choke to adjust POI.

Swamping is part of building a double barrel shotgun. If the bores are straight, and the muzzles are together, thing don't work right. It is just a fact of gun building. Many high grade American guns are swamped to the point where, if the barrels are cut as much as 4", the muzzles are still together, like my wife's DHE Parker #207,672, which is cut 4" from 30" to 26" and still has muzzles together. A.H.Fox provided 26" open bored guns for it's customers by cutting already completed barrels. This is mentioned on factory order cards. I believe Hunter Arms did the same thing. Some barrels from these companies did not have "together" tubes at the muzzle in original barrel lengths, even in barrels that are not particularly short.

Another thing a lot of folks forget is the charge does not hit where the bbl was pointed prior to firing. The above mentioned L C Smith Plans & Specification book has a drawing showing the "CONVERGENCE" of the bbls in building. They further show the point at which a continuation of the two axis would cross & the amount those lines would be apart at 40 yds.

After acquiring this book when it firast came on the market some years back I checkede the doubles I had at that time, they all had very similiar convergence to the Smith Specs. I was of course working from the bbls ODs by measuring their dias & then across their outer sides.

Looking down the bores at a shadow line ( the method used by old time bbl makers for straightening bbls) did not show any of the them to have enough "Swamp" for me to detect. A pair of bbls which converged from the breech toward the muzzle & then turned parallel for the last 4" would be so easy to pick up a blind man could see it. No gun I owned fit that description.

I did not then own, & don't now, a .410 to check. It is quite possible that some of these .410s with wide a breech spacing & touching muzzles are swamped. I don't see how they could shoot to point of aim if not.

Do note though if you "Bore Sight" a double the two bbls won't sight to the same place, I "Guarantee". Their axises will cross not too very far from the muzzles.

As Miller alludes, recoil will cause the gun to rotate laterally in the direction of the barrel being fired, resulting in the payload to exit the barrel after movement and hopefully pointed at the target.

I'm seeing about .011-.013 convergence on my 410s. I would expect to see that on all 410s that hit correctly. Some may have to be swamped to get the latter part of the barrels to that convergence.

If you look at the muzzles on the 410 CSM OO frame Repro barrels for 28g, you see they are about 1/8" apart. No doubt to get to the right convergence
without swamping.
http://parkerguns.org/forums/showthread.php?t=5889

Thank you gentlemen, great information that is much appreciated!
Rick