Where Do Rifles Shoot By Themselves?

[lysanderxiii]

Another source of barrel motion:

The bullet CG is never exactly on the geometric centerline of the bullet, and the bullet's geometric centerline is never exactly in the center of the bore centerline, so, as the bullet spins, there is a radial force spiralling down the barrel pushing the barrel off its "pre-shot" axis. Assuming we have a 'free-floated' barrel with the stock held rigidly and unmoving, this force, as it moves further away from the receiver, exerts a larger bending moment on the barrel, causing the muzzle to move in larger and larger circles, until bullet exit.

 

[jmorris]

Much of this discussion only serves to prove how critical consistency on the part of the shooter is.

In a thread that was intending to ignore human input.

 

[Bart B.]

The bullet CG is never exactly on the geometric centerline of the bullet, and the bullet's geometric centerline is never exactly in the center of the bore centerline, so, as the bullet spins, there is a radial force spiralling down the barrel pushing the barrel off its "pre-shot" axis.

A friend was able to spin some bullets with a Dremel Moto Tool at 30,000 rpm, the unbalanced ones made the tool draw more current. About half tested seemed perfectly balanced to test. He was able to shoot some 10-shot groups under 1/6th MOA at 600 yards. Other groups under 1/4 MOA. Smallest was about 1/9th MOA. Dead calm air just before sunrise.

 

[JohnKSa]

That assumes the barrel is a perfectly rigid beam, with absolutely no flex, which is impossible given the barrel is made from an elastic material.

So you believe you have found a practical problem with my theoretical scenario? Hmmm...

 

[lysanderxiii]

I think some believe a rifle can be sighted in with a collimator in the barrel. Just zero the sight on its reference then adjust point of aim down to compensate for bullet drop and sight height.

Depending on your accuracy requirement is, that might be perfectly adequate.

A friend was able to spin some bullets with a Dremel Moto Tool at 30,000 rpm, the unbalanced ones made the tool draw more current. About half tested seemed perfectly balanced to test. He was able to shoot some 10-shot groups under 1/6th MOA at 600 yards. Other groups under 1/4 MOA. Smallest was about 1/9th MOA. Dead calm air just before sunrise.

A ten sot group from the same rifle in its fixed mount, about as close to the theoretical mount envisioned in the original post:

This shows that error (jump) due to barrel motion (the top three plots) is very predictable. The CG and aerodynamic jump components are caused by variations in the bullets' actual shape and construction. These are the major contributors to the dispersion.

 

[Bart B.]

Having put an optical collimater in a lot of muzzles of rifles zeroed at ranges up to 300 yards, none had their reticle below the target center equal to bullet drop plus sight height. All were off 1 to 2 MOA horizontally and as much or more in elevation. Even double rifle bore axes are several MOA off horizontally from the line of sight as they cross 20 to 40 yards down range.

 

[jmorris]

Having put an optical collimater in a lot of muzzles...All were off 1 to 2 MOA horizontally and as much or more in elevation.

That's actually not too bad, I always though of them like looking down the barrel, good enough to get you on paper. That is more accurate than changing shooters from your earlier post.

I once watched a dozen people shoot a 5-shot group with the same ammo in the same rifle resting it on one or two bags on a bench top. Groups ranged from 3/4" to almost 2". A plot of all groups centers measured 2-1/2".

 

[Bart B.]

A good way to find out if your rifle barrel wiggles from shots being fired, smack it with a hard object. If the rifle rings with a high pitched sound, it's wiggling. If you don't hear any sound, its still and not wiggling.

 

[lysanderxiii]

Having put an optical collimater in a lot of muzzles of rifles zeroed at ranges up to 300 yards, none had their reticle below the target center equal to bullet drop plus sight height. All were off 1 to 2 MOA horizontally and as much or more in elevation. Even double rifle bore axes are several MOA off horizontally from the line of sight as they cross 20 to 40 yards down range.

If you look at the image above your post, and look closely at the first plot.

That is the static pointing angle, the point on the target the muzzle is pointing at, after the bore was boresighted to the aimpoint, but after the laser boresight device was removed. This shows that the weight of the boresighting device affects the barrel to some degree. It even shows up on the Mann barrel in the test, but to a much lesser degree. The pointing angle and the crossing velocity ration also show that there is a fairly consistent difference in the aimpoint and the boresight at the time of bullet exit. The CG jump seems generally consistent in direction and magnitude. All of this tends to confirm what you have noticed empirically.

But, like I said, if all you were going for was 6 to 7 MOA (a shot in the vitals of a small deer at 200 yards), a laser boresight and a calculated drop would be "good enough."

 

[Bart B.]

Rest a rifle whose sight is zeroed for its ammo at 100 yards on bags.

My original setup was the rifle resting on bags. Not fixed nor clamped in a fixed, immovable way. Free to recoil in any direction.