Point of impact

[JudgeBAC]

Is the following observation a factor of velocity, RPM, a combination of the two, or some other factor? My model 29-10 6.5" bbl with a 255 gr. Keith bullet over 9 gr. of Unique places these 1" above the sights at 25 yards and to a similar place at 50 yards. A 277 gr. MM bullet over 9 gr of Unique places the bullets 3.5 to 4" LEFT of the sights at the same distances. What gives?

[Shuz]

JudgeBAC--I've experienced the same thing down thru the years with different boolit weights and profiles shot outta the same revolver(s). I've also been "lucky" that each design shoots consistently to the same place. Since this is the case, I just move my rear sight so that I'm hitting where I want. Seems to have resolved the "problem", but I'll prolly never know why it happens!!--Shuz

[Wayne Dobbs]

A gun barrel is nothing more than a tuning fork with a hole in it. When the weapon is fired, the barrel vibrates at some given frequency and amplitude based on the load. Each and every different load will generate different values of frequency and amplitude. The muzzle end is therefore in different locations during the vibration cycle and releases the bullet at those different points which can cause different points of impact for different loads. The effect is more pronounced in rifle barrels, but pistols/revolvers do it too.

[freedom475]

Torque is what causes the left hand boolit drift.

The boolit is trying to "straighten" the rifling if you will. With the right hand twist of most guns this is causing the gun to torque left. This is compounded by the fact that most of us are right handed. We have a whole hand on the right and only a thumb on the left side of the gun.

I know we have the off hand also holding the gun in most cases but with HEAVY loads the "off" hand is sometimes blown completely from the gun

This is increased when we go up in boolit weight. A heavy boolit, besides having more "torque power," is generally going slower witch results in more time in the barrell again increasing the drift factor.

I can really see this with my 475linebaugh. It seems to be even worse if my focus is on the target and not on the front sight where its supose to be.

hope this helps with your ?

Freedom475:Fire:

[Lloyd Smale]

Its a combo of all these things. Ive even shot two bullet a that weighted the same and were of a differnt design with the same exact load and one will hit 3 inches to the side of another. What i have noticed in the past is if i have a gun shooting to point of aim with a few differnt loads and i try one that shoots off to the side 9 times out of 10 that bullet isnt going to shoot well in the gun. I also tell people trying to regulate a new fixed sighted gun to try some diffent bullet designs before twisting a barrel as changing bullets sometimes is all it takes.

[44man]

It is very common and Lloyd's as are the other answers, are right. I have been pretty succesful with fixed sight revolvers by working loads and powders to move the POI to the sights. I have not found a change in accuracy with loads that shoot to the side once I find the boolit's accuracy point. That is when I just adjust my red dot to the boolit but if you have fixed sights and find a super accurate load that does this, you still need to work on the POI and might have to accept a load not as accurate. You should be able to do it with almost any boolit though. You have to alter barrel whip, vibration, velocity and spin rate.
If the sights are adjustable and the load is accurate, best to just change the sights and record the clicks so you can move from one load to the other.

[happy7]

Ok, this is something I am working on. I have several revolvers with non adjustable, non driftable sights. So from the above posts I think I understand that changing the POI horizontally is a matter of trying different bullets and powder charges till one works but it is not predictable. But for vertical adjustment, changes in impact are more predictable, right?

For instance, a heavier bullet would move the group up or down?

And increasing powder charge would move the group up or down?

What would sizing larger or smaller do?

[happy7]

I have a colt police positive in 38. I have tried it with a 105 grain and a 150 grain. It shot about six inches left with both at 25 yards. If i can't find a load that works, is twisting the barrel the best solution. What kinds of things could make a revolver shoot so far left? It grouped about three inches.

[44man]

Up and down is caused by 2 problems; either the wrong front sight from the factory (very common) or barrel time. Handgun elevation is dependant on recoil. A slow heavy boolit will hit high because the barrel rises higher before the boolit leaves, heavy recoil can do the same with too loose a grip. Speeding up the boolit can lower the POI. A lighter and faster boolit will hit lower.
Not set in stone because other factors can change this.
Side to side can be worked out with powder, boolit and velocity changes unless the barrel just wants to shoot that way. It happens! I had a new Ruger Mark II that hit the ground 20 feet in front of me. I could hold it out level and watch the bullets dive into the ground. Inspecting and measuring everything showed nothing wrong, crown was perfect and the bore was straight and centered in the barrel. No rhyme or reason. Ruger never told me what they found.
I have moved the POI left with a revolver a good 10" at 25 yd's by just changing powders.
Try everything before twisting the barrel.