doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Gun reboring and regulating.

Stan: The purpose of arching those barrels back to back is so that the shot charge follows a line parallel to the line of the rib at the time the shot is fired. After the right barrel is fired the recoil causes the barrels to rotate to the right. The rate of this rotation is determined by the cartridge and the MOI of the gun. So the barrel is rotating to the right as the shot charge goes down the barrel. The intention is that when the shot charge comes out of the muzzle it is moving a line parallel to the rib at the time the shot was fired.

Parker built guns in OOO, OO, O, 1/2, 1, 1-1/2, 2, 3, 4, 5, 6, 7, and 8. One of the dimensions that varied with frame size was the distance between the firing pins. Parker made 12 gauge guns on #2 frames that had barrels 26" long and 34" long. Parker factory barrels always touched. There may be a few exceptions to the rule but I have never seen one. Geometry dictates that the barrels converge at a faster rate on the 26" barreled 2 frame gun than they did on the 34" gun. If those tubes are absolute straight then either the 26" gun is going to shoot cross-eyed or the 34" gun is going to shoot wall-eyed or there is a supernatural force acting on the shot charge after it leaves the barrel.

Read Coosa's post again. In the cheap gun he regulated with a file all he did was put curve in in the last few inches of the barrels by opening the outside half of the muzzles to stop it from shooting cross eyed. After he got through regulating them they shot to a point with the turkey loads but not as good with the lighter loads.

I concede that many and maybe most Fox muzzles don't touch. Same with LeFever. But every English gun I own has touching muzzles.

Miller I would be interested in a more detailed critique of my Parker post. For instance you might explain what you think it is that causes both tubes on the 26" 2 frame Parker to shoot to a point at 35 yards and both tubes on the 32" 2 frame Parker to shoot to a point at 35 yards. I mean in the context of them being absolutely straight.

Miller I don't know what you think you can tell about by straightness by measuring the outside of barrels. Having measured the wall thicknesses on many, many barrels the bores are not necessarily concentric with the outside of the barrel. Frequently the wall thickness on the top of the barrel is thinner than the bottom or vice versa. And the outside of the barrels I have are curved, not straight. I would also like to know the the tolerance your eye is calibrated to when you are peering down a barrel measuring straightness. Does it measure down to 1/100" bend per foot of barrel? Can it detect down to 1/1000" per foot of barrel? Or does your method have some unknown plus or minus figure? Or do you know the plus or minus figure?

I enjoy the give and take of debating this issue. But I am nonplussed when my opponent just declares his position is the Truth because he said so.

Barrel straightening: from what I have read in the past the men who did the work in the factories could detect the slightest crook and correct it. I'm talking closer than .001 for its entire length. Traditional method was to have a thin line drawn on a high window and pointing the barrel at it a shadow of the line was cast inside the bore and any unevenness was readily seen by a skilled workman. This is also the method Skeet's uses. I have watched makers of muzzleloader barrels do the same thing and their ability to straighten them is amazing.

Joe where do you get the .001" ?

And how do you get a straight-tubed two-frame Parker 12 gauge with 26" barrels to shoot both barrels to the same point at 35 yards like your straight-tubed two-frame 12 gauge with 32" barrels?

And I believe you told me the Skeets smith looks for and sees the curve in the SxS barrels so he would know whether to go in from the muzzle or the breech to open up chokes.

I don't doubt barrel smiths can see, feel, and smell things that the rest of us can't. And I don't doubt that they can do amazing things when they are regulating barrels.

Supposedly, visual acuity is a tad better than .0035". Evaluating reflections of a distant line through a tube may very well show distortion that's some multiple smaller than .0035".

If that 26" barrel has more wall thickness at the muzzle, they can touch and have the same convergence as the longer barrels. Maybe so because it might be easier to lop a few inches of the muzzle end of a standard parts bin barrel than special machine shorter tubes. Tubes might not be likely to be shortened from the chamber end.

Maybe the convergence is slightly more for the shorter tubes, but it's too hard to evaluate a tiny shift in POI on the pattern board. Maybe the shorter barrel set has more muzzle flip than the longer ones and more needs to be figured in to end up with a similar POI as the 32" set.

I'd figure, once it's known that wall thickness varies, that could well be enough machining to distort a tube. One of the by products of manufacturing (finishing) that might alter an intentional bend or straightness. The eccentric choke example maybe one of altering flow through the opening than creating a curve in the tube. It could be distance sensitive.

Still hard for me to imaging that an intentional bend was created, visualized, aligned and preserved through the manufacturing process.

Mike;
First & foremost I am not trying to convince anyone that this is So "Because I said So". no indeed, in fact if only my meager evidence were available I would never even have posted on it. What I am saying is the evidence is out there, one simpy has to seek it out. My meager checking simply DID, fall exactly in line with available published data. As to the difference in a 26" barrel & a 30" barrel, not having a Parker at the present time I will fall back to the LC Smith data I posted earlier. Breech spacing was 1.156" with muzzle @ .8175". With barrels set "Straight" & distance between the projected axis of the two bores at 40 yds was 15.431"
For a comparision lets use those exact same dimensions "Except" for a 26" barrel. Convergence from breech to muzzle is still .3385" (1.156-.8175") .3385/26" = .01301923" convergence per inch as opposed to .011283333" for a 30" barrel. Multiplying this by 40 Yds x 36" per yard 18.748". Subtract from this the .8175" from which they started in the opposite direction gives 17.930" or a "WHOPPING" 2.477" difference Re the difference in barrel length only & this is at 40 yds for the total spacing between the two barrels. Error from point of aim would be half that. Also of course this does not give the exact point that gun is going to center its load in firing, but is simply the mathematical extension of the bore axis from a static position. That is where the bores are pointing "Before" the triggers are pulled.
This is all "Hard & Fast DATA, not worked out by me but by Gunmakers, simpy cited by me. Still looking for anything at all from your end other than Pure Speculation & Hear-Say.

Barrel straightening . . . It's quite common, after the first couple morning drives on a driven shoot in the UK, to be offered a nip of what they refer to as "barrel straightener". Depending on the "gun" in question, it can be helpful.

Mike, I understand all the physics behind why convergence is necessary. What you are saying, tho', is that the reason for convergence does not occur until after the shot charge leaves the barrel. Recoil is the reason. And that recoil begins to act on the gun, pulling it to one side or the other, BEFORE the shot charge leaves the barrel. If it didn't, convergence wouldn't be necessary. Recoil from firing the left barrel pulls the muzzles left, recoil from the right barrel pulls the barrels right. Now, stop and picture this a second. If the bores were parallel at, and near, the muzzle, as you say they are from bending the barrels, convergence of the axis of the two bores would not be necessary, because it would be counter-acted by the parallel section near the muzzles. I mean, why in the world would a barrel man put two barrels together with a converging angle, which is to counteract the recoil which pulls the gun to one side or the other, then make the bores parallel? That would be self defeating.

SRH

Stan I don't think we are disagreeing on why SxSs need convergence but I think this might settle whether we do or not.

Suppose my shooting student Joe Wood and I are out dove hunting. A dove flies up and lands on the barbwire fence, to the right of a decoy. (That is Joe's favorite shot by the way). This sitting dove is 35 yards away. And suppose Joe is shooting his two-frame Parker 12 with 32" barrels and extra full chokes. Now, for the sake of argument, suppose the tubes are perfectly straight as advocated by you and Miller. Joe takes careful aim at the dove, setting it right on top of the bead. The rib points exactly at the dove. Since the barrels converge the right barrel is pointing to the left of the dove. Joe pulls the right trigger, the firing pin hits the primer, the powder ignites, the shot starts down the barrel. Because of the recoil the gun and thus the right barrel rotate to the right, approximately around the buttplate. As the barrel is rotating to the right the shot charge is coming down the barrel. Just as the shot charge exits the muzzle it is headed parallel and a little to the right of the line the rib was on before Joe pulled the trigger. The shot, except for vertical curve, goes straight to the dove and it falls to the ground dead. Joe yells "YEEEE-HAAAAW" and does his victory dance.

Now, same scenario, except Joe pulls the trigger and the gun fails to fire. He pulls the left trigger and it also fails to fire. Joe opens the gun and discovers he forgot to put the hammers back in it when he worked on it the night before. I hand Joe my two-frame Parker 12 gauge with 26" tightly choked barrels. And for the sake of argument lets say it had straight tubes as you and Miller contend. Joe aims carefully at the dove on the barbwire. The rib is pointed exactly at the dove. Because of convergence the right barrel is pointed to the left of the dove. It is pointed even further to the left than the 32" barrels because the 26" barrels have a higher rate of convergence. Joe pulls the right trigger. The recoil causes the gun and thus the barrels to rotate to the right as the shot charge goes down the barrel. But since the convergence rate of the 26" barrels is higher than the 32" barrels when the shot charge leaves the muzzle its direction, instead of parallel to the original line of the rib (before the Joe pulled the trigger), is at an angle to the original ribline. In a few yards the shot charges crosses the original rib line and goes to the left of the sitting dove, hitting the decoy and knocking it down from the wire. Joe yells "YEEEE-HAAAAW" and does his victory dance. The dove looks around, shakes its tail, and flies off.

Stan I am not contending that the back to back bananas give parallel bores. I am contending that curved tubes are part of SxS barrel regulation. Coosa's cheap gun could have been regulated by putting curve (more curve) in the tubes when they being were assembled into barrels at the factory. Coosa put the requisite bend in with a file in the last few inches of those tubes to regulate them properly. Had he been so inclined and had the requisite skills he could have taken the ribs and bulkheads off and put curve (more curve) into the tubes and then reassembled them to get the same result.

All that is cool, Mike. Except that the makers took into consideration the fact that short barrels would need a different angle of convergence to have them truly regulated. You are assuming, I assume ( ), that all barrels, whether 26" or 32" are touching at the muzzles, and they are not. You are also assuming that all of them will center their pattern perfectly at the distance the dove was sitting on the fence, which they also do not. And furthermore, that Joe's extra full pattern is not going to be big enough to kill that dove on the edge of the pattern, even if it IS off because of the differing angles you speak of.

This is not all exact science. Bench rest rifles are, shotguns are not. As I alluded to earlier, there are probably more double guns that are NOT perfectly regulated, than there are those which are. The angle of convergence is calculated to be right with what the maker concludes will be the most likely load used in it. Loads on either end of the spectrum, heavier recoiling or lighter recoiling, will affect this.

One more time, why would a maker go to the trouble to build an exact angle of convergence into the barrels of a S x S and then bend them to parallel at the last few inches?

SRH