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

Miller,
I did that conversion to 28g and it would cross POI until I really lowered the convergence to something like .012 from memory. I'll have to check them to know for sure.

I have a book named "PARKER America'. Finest Shotgun" by Peter H. Johnson. In a section he titles targeting, he explains Parker's process for targeting their barrels.

All the barrels were sent to the "shooting room" where they were locked into a fixed rest, aimed, and fired at a 30 inch diameter steel plate at 40 yards. The barrels were re-bored or straightened until they patterned dead center at 40 yards.

As stated above, I doubt this much effort is put into any gun currently made. Also, technology should have come up with something more efficient by now. However, I think this is a good example of how doubles should be set up to shoot. Correct terminology or not, this is what I visualize as convergence.

If the two patterns are seven inches apart at 16 yards, there is something definitely wrong, at least in my opinion. I could be the regulation, or the choke. Using the Parker method as an example, I would think an aimed pattern would center just right and left of your aiming point at 16 yards from the respective barrels.

Hope this helps,

Milton

I have a gun that when patterned at 16 yds showed that the left bbl shot slightly TO the left. About 1/2 a Pattern size. I noticed this while shooting clays, smoking birds with the right bbl, and breaking them with the left. I havnt decided if this is enough to mess with yet...Can this be corrected by lightly honing the choke on one side?

Jim
Not sure I agree, if you think of the width of the tubes at the muzzle vs the breach a straight line down the bores would cross shortly after exiting the tubes. I think the "magic" is that they shoot close together at the 30 to 60 yard distance. I know that Oscar used to take great care to replace the packing in Parker barrels at the same spot it came from originally and that he also was very careful in the allignment of the end of the tubes. I think there is some regulating going on with a side by side but not to the extream of a side by side rifle.
bill

Just for a ROM (rough order of magnitude) check, say the measured convergence of the barrels is hypothetically .020". In 10 inches downrange that's .200". In 100 inches (about 8 1/3 feet or less than 3 yrds) downrange that's 2". In 1000 inches (83 feet or 28 yrds)that's 20 inches. Given that the barrels are only about an inch apart (centerline) or more like 3/4" at the muzzles, the gun should theoretically shoot crossed patterns about 19" apart at their centers at 28 yrds, or about a foot crossed patterns at the 16 yrd line.

I chose .020 since I know my little guns are less than this and the big bores are more. But this should illustrate that there are dynamics involved that simple trigonometry doesn't account for.

LD, yes it can. I think I saw pictures of the method being applied in one of
Gough Thomas' (G.T. Garwood) books. Can't remember which.

JC

Convergence can be changed most of the time by selective honing one side of the choke. Think about it this way. As little as a .020 constriction on both sides can make a 50" wide pattern at 40 yards into a 30" pattern. So selective altering one side will change the pattern by several inches. You do get looser pattern when you do this, at least I did.

Figure each .001 should move the pattern about 1/2-3/4" at 40 yards. This is just a guess from two guns I had done. You have to pattern them to make sure that you are getting the change in impact that you expect. First "gun smith" did the wrong side first and we had to do it twice to move the pattern 6" to the right. I guess we went form a full to a improved choke because of the error.

I have read that most double barrels were set to converge at 40 yards. This make sense if you figure that guns were set up to be used at that range. If a skeet gun then you would expect more like 21 yards.

I didn't think that, in an ideal world, patterns should converge or cross over? The idea of barrels converging (in a sxs, not o/u) is that the recoil, because it is not directed in a straight line back to the butt, pushes the shot from the left barrel to the left, and right to the right, so that the convergence is to correct that - just like in a DR? I thought it shouldn't matter at what distance the gun is to be shot - the pattern should still be on PoA for a properly regulated gun?
RG

O/U bbls converge also, if you check the distance from CL's at both breech & muzzle. Otherwise the top bbl wouild shoot much higher than the lower one due to a more rapid lift from recoil.(higher above point of contact with shoulder). Regulation is a factor in both SxS & O/U.

Cadet,
Sorta yes and no. Since the recoil of the barrel moves the gun laterally, the shot exits the barrel as the barrel comes into alignment with the point of impact(hopefully also the same as point of aim). This means the shot actually came out of the barrel from a certain distance laterally from the aiming line of your eye-front bead-target. Granted this lateral displacement is small, but it exists. If we assume the gun rotates laterally about the butt of the stock, using the measured convergence we can calculate the distance the muzzle would have to move laterally to be pointing at a target at a given distance.

If we use the .020 convergence which came out to about a 12" crossed shot at 16yds, and used 50" for the gun length, the muzzle would have to move about 1" laterally to line up with the target. At 32yds, the shot would be 1" crossed over....in theory. Could you see any of this??? Unlikely, IMO...