Help me understand the ballistics here...

[richl025]

Testing out different factory loads in my brand new Tikka .308...

I know different rifles just seem to prefer different loads (for no good reason), so I bought a few different loads to test.

I can completely understand why some groups are wider than others, but why the hell is the bottom group clustering several inches to the left? Slightly high or low I can understand as a function of velocity, but what is happening to this one load that is causing it to group to the left? Are the bullets spiralling like a football? (And if I had shot out to 200m it might be to the right then?) Or are these bullets literally exiting the barrel with a slightly leftward trajectory?

This is more of an academic question, since obviously I chose the cartridges from the middle group, but this aroused my curiosity, and I wanted to see if anyone has a good explanation

(FTR, these are 6 shot groups at 100m - five shots fired in a slow series at each target in series, and then a final shot with each of the tested cartridges at the end to try & control for barrel heating and/or fouling. Yes, I am a crappy rifle shooter, and the size of the groups is probably at least partially my poor form...)

 

[ozyclint]

Barrel harmonics?

I have played with reduced loads in my .222 and it shoots a little left with those too.

 

[XLR]

Different bullet weights, powder charges, etc will all effect your harmonics and where the bullets are impacting. For example when you put a suppressor on it doesnt always effect just vertical, it can change your zero horizontally too.

 

[HandgunHTR]

Yep. Watch a super slow-mo video of barrel whip. You will see the muzzle moves in a circular pattern, not just up and down.

Your form will also dictate changes in point of impact. Were all of those loads using the same weight of bullet?

 

[EastHumboldt]

I’ve had similar experiences with handloads in 7 MM 08 and 30-06, both tikkas. Variations in powder charge with the same projectile tend to create vertical POI differences. Sometimes a different projectile will tend to move POI horizontally, or both horizontally and vertically. I would attribute it to harmonics.