doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Cast check - POI change

The purpose of cast is to align the rear sight (eye) with the sighting line. Some need cast others don't, but the eye as any other rear sight, when moved a particular direction, makes the gun shoot that direction, to some amount. Take in to account the shooter's face, the taper of the stock and the angles before and after the bending. My impression is that for two barrels to impact so differently and it not be a regulation issue, the stock fit must be so poor that the gun is moving alot when mounted and assuming it is a twin trigger gun, the position of the hand factors as well. Onc e the gun is on the shoulder and to the face the alignment should be correct and both barrels should impact simularly, of course. For this not to happen, the barrels are not regulated or the position of the gun is changing. Bending the stock may help with the later.

Kurt,
The gun is a single trigger.

I have not excluded that there is a regulation issue.

I am starting to question whether cast can create problems with lateral POI shift that are undesireable.

Given that it is crossfiring, If I simply loosen the rib/barrels and spread them, I should be able get them to both impact the same, but they would be left of point of aim down the rib. I'd still need to move POI to the right. I'd just like to do that first, then deal with any regulating.

Chuck, Without doing anything with barrel regulation (spreading the barrels), to get the right barrel to shoot more right, the shooter's rear eye must be moved to the right. That requires more cast off or less wood on the left side of the stock. If the shooter uses the front sight, move it left or shape it left. You may just try taking it off? A combination to try before you widen the barrels. The choke can be changed too.

So now you tell me.

Chuck, IMO, the POI will move in the same direction as the cast or drop is moved; cast-on (move to the right) increase moves POI right and more drop moves (move down) POI down. Unless there is a handling/trigger pull issue, stock adjustments will not change differences between POI of the two barrels for a given POA. You can center the current spread in POI with stock adjustment. However, I'd strongly recomment that you deal with regulation first and then adjust the stock to a common, between the two barrels, POI. You can't know before hand where the common point is going to be. So, adjusting the stock to a central point does not assure that the regulated POI will be the central point. You could easily find yourself re-adjusting the stock all over again.

Back up a couple of steps here. You haven't given complete information yet. For example at what distance did you test this, and how far apart are the points of impact for the barrels. I suggest you fire this gun from a supported standing position, several shots from each barrel, and use an aiming point on the target. Sort of like bench resting a rifle. Use an experienced shooter other than your brother. Find out if the problem is the gun or shooter, find out if the spread between the two barrels is shooter induced or a gun problem. Then you can start working on the gun issues if needed. If the barrels need regulated do that before you start bending the wood. Then if needed you can bend the wood.

I had forgotten my Michael Yardley "Gunfitting" book until early this morning. I pulled it out and read a bit on cast. He quotes Gough Thomas who says that POI moves opposite to the direction of the butt movement.

Regarding the distance tested and how much the barrel was off point of aim, it was shot many times at 20 yrds and was off approx 8-10" but I didn't have a proper measuring device.

Straight trig change would suggest that a change of over .300" change in barrel spread would be needed to correct this. Something is off big time. I'll check the barrels for a converging bend, then look at the chokes for proper alignment.

Again, the left barrel was spot on.

I've check my brother-in-law's mount and sight picture by having him repeatedly mount the gun and I peered directly from the muzzles down the rib to his eye. he has it mounted correctly with a correct sight picture.

As i said before there are two factors at work here. Quite obviously change in cast moves the position of the eye in relation to the pointing of the gun. Moving the eye is just like moving a rear sight, the shot will shift in the direction of the movement.

The other factor is movement of the gun itself while the shot travels down the bore. The fact it does move "Sideways" is brought out by the fact the bbls are not set parallel, but converging. Figuring that the entire gun is pivoting about the point of stock contact with the shoulder, then moving the butt will cause shot to shift in an opposite direction.

I personally think, but don't have proof, the change of sight is going to be larger than change of gun movement, thus impact is more apt to shift in direction of movement.

It wouldn't be the first time Gough got it Wrong.

Try a heavier load in the right barrel.

Nothing wrong with Thomas's bent lever theory--cast off combined with recoil forces should move muzzles and POI to the left for both barrels, perhaps more for the left barrel. Perhaps as Pipes states, this recoil offset isn't appreciable relative to back site movement created by casting the stock.

jack