Point of Impact Changes

[Prose]

Why does POI change between different bullets? Most of my rifles have little or no change. For instance my 20VT will shoot Nosler, Hornady, and Sierra bullets within 1/4" of each other. My custom 260 Rem will shoot everything into the same spot at 100 yards no matter the weight, shape, or make of bullet.

However, my wife's 308 win (Douglas 1/10 generally about 1/2MOA @ 100 yards) will shift 8-12 inches left right up down. 150 Hornady Interlock SP shoot 10 inches right. 150 Sierra Pro Hunter Flat Point 10 inches high and a couple to the right. 150 Sierra Prohunter Spire Point dead center. 150 Sierra gameking 6" left. Speers are low. Extreme spread left to right is 1 1/2 FEET. All bullets group fantastic though just got to adjust the scope is all. All loads tested for accuracy noads ect. Barrel cleanliness or fouling is not a factor here.

I'm familiar with barrel whip/harmonics ect, but never really though of a barrel whipping in a circular motion. As I ponder this kinda makes sense as it is a savage 99 and has a very small shank of minimal diameter that could be more influenced by a tight twist. This is a featherweight barrel, 22". Ya ya just shoot one bullet I know. Probably nothing could fix the situation, just trying to understand it better.

 

[Prose]

Oh I should note that Point of Impact per bullet remains the same despite powder charge/velocity.

 

[T-shooter]

I've never seen anything shoot that far off. But I was testing bullets this year. At 215 yards the 208g A-max, 168g Sierra M/K and Hornady Match 168g all hit about the same point of aim, the A-max about 2" low due to the slower velocity. Windage was the same. I started testing Berger 200.20x and they all shot 4"+ to the right and 4"+ lower. This was with a 200g vs a 208g bullet, the same powder charge, and similar BC. The A-max groups well at 2,430 fps, a maximum load. I took the Berger up in steps to 2,575 fps and still shot lower and to the right. As the velocity changed, the elevation didn't for whatever reason. The A-Max (never worked the load) averaged .6 MOA and the Berger averaged under 1/2 MOA so far. Like you said, just adjust the scope for the difference.

 

[Papa Charlie]

There are many factors that will affect the POI. Barrel whip is a factor as is the action, bedding, bolt alignment, etc. The whip effect is not just a sign wave, you also have the affect of the bullet spinning down the barrel as well with the interference fit of the lands into the bullet.
Every rifle is different, so all these have different affects depending on the rifle. Some may have little affect while other may experience more. The affect of all of these factors can be exaggerated by the weight of the bullet as it is spun down the barrel.

 

[FocusGroup]

I was going to start a new thread to see if anyone could help me understand this but I have a similar issue. My .243 is shooting dead on at 100 yds but when I put lighter bullets in it even of the same brand my point of impact seems to be not only a bit higher which would be expected but a good bit to the right. It's a 1-10 twist barrel and I'm wondering if the twist is affecting the lighter bullets in some way? Any idea what's going on here?

 

[Whit holman]

I think it is the heavier bullets being marginally stabilized is why when you shoot the lighter bullets that are stabilized properly the point of impact move so far

 

[K22]

Perhaps you're over thinking this a bit - creating a problem where there is none. If the wife's 308 shoots 1/2 moa with load "A" just use load "A". By the way, that's impressive group - don't fix something that isn't broken.

If for some reason you have to change components then re-sight for those components. I would only be concerned if for a given load you are getting erratic groups.

 

[Dusty Stevens]

Barrel harmonics ARE in a circular motion.

 

[Joe Salt]

Try changing the scope see what happens!

Joe Salt

 

[varmintshooter]

Savage M16 in 223 did the same thing. If I shot 3 rounds of each, 50gr, 55, 60, 69 and 75gr bullets, I would have 5 groups. The lite bullets hitting high and left and the heavy hitting low and right. I cut the barrel from 22" to 18" and threaded for a can and now all weight bullets hit in one group. The can is an Elite Iron Delta and weighs 23 oz so I'm sure that has a lot to do with it. My Savage M10 (heavy barrel) would put all bullets in one group. Both were 9 twist.

 

[theallcineyes]

Every barrel is different. I suggest that when you buy a case, you use a box to verify and record zero for future reference.

 

[FocusGroup]

Thanks for the input. Yeah, load A works so I'm just using load A, specifically Winchester Super X 95 grain loads. However, I was puzzling about the physics of what would make the lighter load point of impact shift to the right. Now that's kind of weird that varmintshooter got different loads to shoot the same by shortening the barrell and putting on a can. Would really like to know what went on there. Maybe the muzzle blast was a factor that got tamed down?

 

[D-4297]

Any time you attach anything of any weight to a barrel , You change the harmonic oscillation and wave-lengths . Even something as light as a V-3 chrono . By adding the "Can" , after shortening the barrel , he radically changed the "wave" , and oscillation of the barrel . A fuller understanding of all these issues can be gained by reading Brian Litz books regarding ballistics for long range . Add bullet weight , bearing surface , velocity , twist rate , barrel length , type - 4 , 5 , 6 lands , 5r , 6r and you really are trying to answer a very complex issue with short-hand answers .