POI question while changing ammo (lateral shift?)

[MyRoad]

I understand that if I change ammo, it can change the point of impact (POI). When shooting .308 if I change from 147gr to 162gr there is usually a noticeable drop based on the bullet weight.

Today I had a different phenomenon when I changed ammo. The short story (I'll fill in the details below) is that I zeroed my new scope with one ammo, decided to test another, and the group moved 5" to the left (not "stringing", but a definite group in a new location). Huh? I can understand vertical movement based on hotter or cooler loads, or different bullet weights, but moving horizontally?

Can anyone explain why this might happen? Is there anything "normal" about this?

The details, for anyone interested: I was shooting a DSA SA58 Para 18", with a 1-4x AccuPoint scope. I zeroed it out to 100 yards with 147gr. Lithuanian 2004 manufactured non-corrosive brass cased ammo. It was a cold very foggy day, I was shooting at a 10" ring/target with a 5" shoot-n-see in the middle. All things considered, I was satisfied that the groups were about 4" at 100 yards (staying within the s-n-s). I changed over to some stuff I got from ammoman, that is advertised as "match grade" 168gr. M118 (manufactured by Igman). The groups were a little tighter... and the center of the group was a full 5-6" to the left, and 1-2" higher. Up and down I get, side to side I don't.

 

[nastynatesfish]

i had the same thing happen when i started loading for my fal. i started out with 180gr mil surp and got it sighted at 100yrd and about 2'' then started loading and i had some 165 hor interbonds laying extra. when i shot them they were high left too. it dont know if it has something to do with the barrel resonance or what but i settled on nosler 168 match hollow points and they shoot awesome. i can even use my rear sight adjustment out to 800 yrds and be in the park at least

 

[rcmodel]

Barrel harmonics.

It vibrates like a tuning fork every time you fire it.
Where it is in the cycle when the bullet exits the muzzle is where the bullet goes.

The same bullets vibrate the same way every time, or nearly so.

A different bullet weight or style makes if vibrate a different way.
Nothing says it must vibrate only up & down.
It can just as easily vibrate sideways.

http://www.shootingsoftware.com/barrel.htm

rc

 

[MyRoad]

Thanks so much for the information! That explains a lot -- I really appreciate it.

 

[JDGray]

I love it when they viberate into the same hole