doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Barrel regulation - how close is close enough?

Chuck H - I guess this barrel regulation stuff is harder for me to understand than I thought:

I am just trying to make sure I understand you when you say:" . . . 15-20% of the pattern diameter would be a limit." and then say "It doesn't matter what choke size we're talking about." Are you suggesting that pattern diameter doesn't change with choke size? Maybe I am just getting tripped up by symantics? Your point that large bore shotguns are more tolerant of poor barrel regulation than small bore guns certainly makes sense to me.

I know that choke size can be said to be the percentage of pellets within a 30" dimater circle at 40 yds, but it also seems to me that pattern diameter does change with choke size (see for example question no. 4 and associated table in this article by Layne Simpson http://hunting.about.com/od/shotgun/l/aastshotgunfaqa.htm ).

Just from a seat-of-the-pants geometry perspective, it seems to me that choke size and barrel regulation deviation are related. For example, a shooter shooting a poorly regulated shotgun may still hit his target with either barrel with cylinder chokes but completely miss it with one or both barrels with full chokes (assuming same distance, same lead, same target speed, etc. Not that shooters normally shoot a SxS with both barrels having the same choke, but it illustrates the point, I think.). In other words, the larger diameter pattern of the cylinder chokes completely overwhelms any POI inconsistencies between barrels due to barrel regulation, where the same can not necessarily be said if shooting full chokes. The same affect would exists when shooting a gun choked Cyl and IC versus Mod and Full, the effect just wouldn't be as obvious.

Just wondering "out loud."

--shinbone

If it helps, think of the centers of the two patterns, not the percentage, chokes or overall diameters. If the pattern centers are together, or close, the gun is regulated to an acceptable degree. When you judge a rifle group, you think of the bullet centers, not the bullet diameter. Same general idea on shotgun patterns. Hope this helps.

Has anyone else found that heavier loads with more recoil may move points of impact right for the right barrel and left for the left barrel? I saw this when patterning 1 oz. and then 1-1/4 oz. loads through the same gun. I didn't follow up by testing with other variations of loads, so I suppose results could be due to other differences in how those barrels digested those particular loads.

Jay

It's been my experience when patterning guns that the right barrel is seldom off; the left barrel center of pattern sometimes is 1-2" left of the POA. Could it be that the right barrel is regulated first and then they try to match the left?

I also believe if the stock butt isn't squarely in the shoulder pocket, patterns will be off because of side torque.

Gee Shinbone, it's not just you that's getting tripped up by ''semantics and seat of the pants geometry". A lot of people on here seem to have no idea how gun barrels are put together. They aren't regulated. They aren't shot for POI. They are put together in a jig or fixture that is set up to give a convergence point somewhere from 30 to 40 yards depending on the maker and the gauge. That is just the center line of both bores crossing visually, not where two projectiles would cross because projectiles come in various weights and velocities. If you've got the money and can specify shot weight and velocity then gunmakers can produce barrels that will shoot anywhere you want them to. Lacking that they do the next best thing which is to line up the tubes visually. Sometimes things don't go so well and one barrel fires North and the other fires West. If the divergence isn't much it can sometimes be corrected by muzzle work but that's a bit hit or miss, pun intended.

Everyone- Thanks for all the information.

Patriot USA - Your info is expecially helpful. Thanks.

nialmac - Thanks for the info on barrel fabrication. I guess you have a good point that "regulation" isn't quite the correct word in the context of mass produced SxSs since there is no shooting and subsequent adjusting done to the barrels' alignment. Maybe just "barrel convergence" is the best term?

BTW, I noticed that Champlin Firearms offers a double rifle barrel regulation service for $600 plus ammo. Anyone have any experience with this service? I wonder if Mr. Perodeau will also regulate SxSs if one were to buy a nice used SxS with a barrel convergence problem?


--shinbone

Nialmac hit the nail on the head. Virtually none of the guns I've ever been able to afford would have been shot and/or adjusted for regulation. I would only expect such effort from a maker of a very very very expensive gun. The H&H video shows this process. But for us commoners, we're getting the mass produced version which is just cookie-cutter fixture assembled barrels.

Chuck, as memory serves me, the H&H video only showed the regulation of a rifle, not a shotgun. I believe the shotgun work was for pattern only.

The L C Smith Plans & Specifications shows the convergence to which they set their bbls. I would have to get it out to remember for certin but they are set for the bore lines to cross close to the muzzles. As I recall continuance of the axis to 40yds gives a divergence of 12"-15", which would be unaccepctable if it actually shot there. This is of course expected to be offset by the recoil of the gun. As the gun starts to move the instant the charge does & as a heavier load recoils more than a light one, we could not really expect all loads to hit the exact same spot now could we. This is of course not so critical with a shotguns pattern spread as it is with aa rifle. Divergences which would be unacceptable in a rifle may well be acceptble in a shotgun, considering both the pattern spread & the max range it will be used. A 4" divergence @ 40yds would be a 10" one @ 100yds. This would be acceptable in the normal shotgun, but not a rifle.