doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Double Gun Convergence question.

Here's one that's got me thinking. As far as I understand double barrel guns are regulated to converge at a set distance. Does anyone know if there is a standard distance to which they are regulated? I ask because the method of determining POI at 16 yds seems off to me if the barrels are regulated to hit at a different distance. I've not seen anything written on this. Any thoughts?

30 yards for a 410, 40 yards for 28 ga and up. But keep in mind that you are talking about a shot pattern, not a rifle bullet. A gun will not be regulated as closely as a rifle, and in the lower price ranges they will have barrels assembled in a jig and not actually test fired and adjusted.

The barrels have 'convergence' because if they were parallel, the fact that each barrel is off the centerline axis of the stock, causes the recoiling gun to move slightly outward in the direction of the barrel. This is 'corrected' by the 'convergence'. If you doubt this, take a set of sxs barrels that you know to have a good POI for you and set them on a solid surface, point the rib at a target, then look down each bore. They will 'converge' very close in. When you shoot that particular gun, you'll find it hits on point of aim regardless of distance.

So, verifying POI can be done at any distance. It is typically done at 16yds only when trying to calculate 'adjustments'. So, for the average guy just trying to see if your gun hits on for you, shoot it at the expected distance the gun/barrel will be used, IMO. I leave the "proper" POI check distance for when adjustments are known to be needed based on the test at functional distances.

Hi Chuck:

Your theory about the recoiling shotgun barrels brings me to something that I saw in the 50's. Winchester gave away small books with a large amount of charts, pictures, and other forms of visual medial free via gun dealers. One thing that I remember well was the article about recoil. They posted high speed photos of how guns recoiled upon firing. The test was done with a model 94 if I am correct. The pictures showed that there was NO muzzle movement until after the projectile LEFT THE BARREL!

I was too young at the time to understand why this was happening, but I took their word that it was true. After all, there were the photos to prove it was true. If this were true back then, then it still must be true today! Since then I have seen more photos, that supported this idea.

I had it expained by an "expert" that recoil was caused by the air rushing back into the barrel to fill the space vacated by the burning powder and the bulllet. This "expert" also claimed that this was the reason that muzzles became bell shaped after many round had been fired. It was caused by the erosive effect of the air reentering the gun.I did not accept this at the time and still do not. How does one become a gun "expert"?

In any case, Have a great Holiday Season.

Franchi

Major General Julian S Hatcher, Ordanance Dept, US Army, stated a friend once told him the Navy dept had proved a gun did not start to recoil until the bullet had left the bbl. General Hatcher replied;
"Be that as it may, EVERY GUN, Large or Small, Army, Navy, or Civilian, STARTS MOVING BACKWARD AT THE "EXACT INSTANT" that the bullet "STARTS MOVING FORWARD"".
How many proofs does it take?? Revolver barrels point below the target prior to firing, because the bbl rise brings them on target. Dbl shotguns & rifles have their bbls converging because the bore axis is off the C/L so they tip out sideways before the bullet/shot leaves the bbl. Drawings from the L C Smith plans book show a 30" 12ga bbl's axis to cross @ 6ft from muzzle & to be 15.45" apart at 40yds. Understand this is "Not" where the shot hits, but the mechanical extension of the bore centers from a stationary gun. "The Gun Moves" when the projectile moves.`If you really feel like delving into this i would sugest obtaining a copy (And Studying it) of Hatcher's Notebook.
Someone fed you a line.

Hi 2 piper:

my question is, did Hatcher have any "proof" of his theory? If so, who did the study and who paid for it? Perhaps this was just one of his theories!

How could the Winchester lab, H.P.White, get the photos wrong. This was before Dr. Edgerton and his high speed photography became the vogue. I have no idea if spark photograpy was used or not. The pictures looked like convential photos of the time.

In any case, the firearms field is an intresting area of study due to all of the theories, myths, and untruths the seem to prevail. Some are very amusing.

The old adage of believing only half of what you see and none of which you hear should be kept in the back of everybodys mind when discussing firearms. Always think carefully and question all that you "learn" to draw your own ideas. What is fact today may not be fact tomorrow.

As always, I look forward to your posts.

Franchi

The proof has existed for hundreds of years. "IF" the gun & load did not start to move simultaneously the gun would not move at all. The internal pressure pushes "All" directions at once. A gun is more or less simply an internal combustion engine which "Throws" it's piston out the end with every stroke. Long before Hatcher wrote the principal had been "Proved". He simply stated the facts & gave revolver sighting as an example. Since this is a dbl-gun board the bbl convergence of dbls is probably better proof. "IF" the gun did not move all doubles with converging bbls (do you know any which don't) would cross fire to the extent of their convergence.
I find it utterly amazing that winchester & H P White would come out with anything like this.
"For Every Action There is an Equal & Opposite Reaction". To the best of my knowledge this is still in effect.

Hi Miller:

Just had a thought about the revolver moving before the bullet exits thes the need for a high front sight to compensate for the movement. Do you think that this muzzle jump is caused by the escape of gas in the barrel/cylinder gap? Why don't automatic pistols have high front sights? Would they too experience the same situation? What about bore sighting a rifle very carefully. I have done and seen this done where there was almost no need to change the sights or scope. Oh yes, rifles and shotguns do not have high sights to compensate for this barrel flip. I appears that revelovers are in a class by themselves. It must be due to the hog leg design of a revolver. All of the other firearms that I have come in contact also had the barrel above the grip, or centre of mass thus inducing a pivoting or fliping motion to the gun/rifle. Think cheek slap.

Oh my God man! Why would you beleive or repeat anything that a general said? Are you too young to remember all of the lies that the public was told during the war with Vietnam? or today re Iraq.

I do not think that I have all of the answers but I attempt to try to understand some of the myths about firearms. I was also told that the reason that a gun does not move before the bullet exits the barrel was that it was akin to attempting to lift ones self by your own boot straps???????? Perhaps recoil is caused by the jet action of the gas being blown out of the barrel at high pressure????

All that aside, have a great Holiday Season.

Franchi

Franchi;
The bore of a 12ga shotgun is .729". That amounts to an area of .417 SqIn (.729² x Pi/4). If a load peaks at 10,000 psi then for a "Breif" instant there is a rearward push on the gun breech of 4,170 lbs ( more than 2 tons). This is incidently what causes "Recoil Operated" firearms to operate.
"IF" when the charge left the bbl it resulted in pulling a complete vacuum in the bore for the air to "Rush" back into (Not at all likely as the gases are still exiting after the projectile has left) then consider that even at a high barometric pressure of 31.25 inches of mercury atmospheric pressure is about 15.34psi. Again in tha 12ga bbl of .417 SqIn this would result in a force when it hit the breech of about 6.4lbs. The gas thus has over 650 times the force of air pressure.
As to auto pistols remember the slide moves indepently until it comes to a halt at the rear. It thus slides along it's axis as the upward pivot from the grip is not there yet. I pulled the slide from a 9mm auto & it weighed 10.5ozs or 4,593 grains. Figuring a 115gr bullet plus half of a 6gr powder charge (the powder gas fills the bbl so it moves about half way until bullet exit) gives 118 grains. From the base of the bullet to muzzle is about 5". 118/4,593 x 5 = .1285". The slide will have moved just over 1/8" (.125") whn the bullet exits the bbl, but in that movement the momentum which accounts for the recoil has been imparted to the gun.
As to hatcher I have no idea how many lies he may, or may not, have told but this is not among them. All of this is well proven, I quoted him more for "The Quote", which I thought was so well put. Another of his quotes I love was when he said "That sounds good, When you "Say it real fast", but it won't stand up to inspection".
The statement of the recoil being caused by the air rushing back into the bbl is in that exact category. Matter of fact it is "Hotter Air" than even the hot burning gases exiting the bore.
Picking ones self up by their boot laces is of course in no way similar to igniting a charge of powder between "Two" objects (Sounds good though "IF" you say it fast). The powder charge puts an equal force in both directions & "On Both Objects". In W Greeners book "The Gun" 1834 he mentioned putting plugs in from both ends of a bbl with powder charge & touch hole in the middle. Guess what, it blew them both out opposite ends. The reason the charge moves faster & farther than the gun is simply because the gun weighs so much more.

Franchi,
Take a 1911 auto and slip a wooden dowel that fits the bore down the barrel. It'll show you that indeed an auto also has the barrel pointed downward from the sighting plane. The barrel in a 1911 is not parallel with the slide or its rails when in battery. The back of the barrel cams up into the lugs when in battery and drops down to approximately parallel with the slide when the slide is retracted.

As I said earlier, if you doubt this, set your favorite sxs barrels on a table or something, with the rib sighting plane pointed at a selected target. Then look down each bore.

To explain this, I think it's best stated as "for every action, there's an opposite and equal reaction". If a mass (projectile/shot) is accelerated in one direction, the gun encounters a equal force in the opposite direction. Like Miller said, as soon as the projectile starts to move, there is force pushing on the gun in the opposite direction and causing movement.