POI change with different ammo: why?

[ArmedBear]

Anyone know why, exactly, 150, 165 and 180 grain ammo in a .30-06 (for example -- pick any centerfire rifle round with decent velocity) have different points of impact?

What about different brands of seemingly similar ammo?

I understand slight differences in trajectory due to velocity, mass, etc. I'd expect some vertical change in POI, especially at a good distance.

I'm talking about 100 yards, no wind, bolt action. Group size near 1", so I'm also not talking about flyers.

Why does the POI change, say, 2" low and 3" left, or whatever, with slightly diffferent bullets or different manufacturers? What's the science here?

 

[Grump]

Barrel time, recoil, rifling torque and whatever level of resistance there is to those forces.

For an exaggerated view of this, just put a revolver upside-down on a table and see where the barrel is pointed, compared to the sights. The difference in boreline (UP there on the wall if upside down) and sight line (level and parallel to the tabletop) shows that for a right-side-up handgun, the barrel recoils upward to match the line of sight.

Lesser effects for rifles. With a 10-lb .22 LR rifle, I've seen POI differences of 3/8-inch at 50 feet, just by how "heavy" the head is on the stock and how tight the buttplate is to the shoulder. Heavy and tight was lower, light and loose was higher by 3/8" and to the right by about 1 MOA.

 

[Thefabulousfink]

Well, I am by no means a ballistic expert, but I do know a little.

Assuming the same powder chagre, different bullet weights will have different velocities and this will affect their ballistic arc. So changing the bullet weight is going to change the POI up and down at longer ranges.

Also different bullet weights need to spin at different rates to achieve maximum stability (this is a big issue with .223). The twist ratio of the rifling in your barrel might stabilize some bullets, but not others.

As far as different brand of ammo, I have noticed that ammo companies sometimes use slightly different measurements for their ammo. These differences are very small, but can slight changes in consistency.

The characteristics of the bullet and powder charge are very important to where your rifle will shoot it. In order to achieve repeatable accuracy you have to find the ammo that shoot the best for your gun and do not deviate from it.

 

[rbernie]

When you fire a live round in the chamber of a firearm, the barrel and action resonate like a tuning fork in response to the forces applied against them. The resonant frequency will be altered based in part upon the shape of the pressure curve and the overall pressure applied. For example, short/fast pressure spikes will cause different resonations than would a slower and more gradual increase in pressure, even though both may result in the same overall pressure being applied.

Since the barrel is wobbling about its longitudinal axis, it whips the muzzle about (since the muzzle end isn't as constrained in its motion as is the breech end is due to the extra weight of the receiver). Any changes in resonation characteristics (change in loads) causes the barrel to be pointed in slightly different directions as the bullet leaves the muzzle. As a result, the POI varies in windage as well as in elevation in response to changes in loadings.

This is also part of why short/fat barrels and heavy muzzle devices can actually increase accuracy (by helping to dampen low-frequency oscillations) and why longer/thinner barrels are more prone to stringing and such. The Browning BOSS system works to 'tune' these resonant frequencies to allow the user to best match the barrel harmonics to the load under use.