Low, left

[Sistema1927]

I sighted in a new Ruger Gunsite Scout rifle (GSR) today, both with the irons and a Leupold 2.5 LER. All went as expected, using surplus 7.62x51 gave me approx. 3 MOA groups. Transitioned to factory Federal 150 & 180 .308 SP with predictable results and ~2 MOA.

Then decided to try some light loaded rounds that I had previously worked up for my Ishapore 2A1 p-scout. 38 gr. of 4895 pushing a Speer 168 BTHP. Gave me a sweet sub-1 MOA group, but 4" low (not surprising) and 5" left (!?!).

Any ideas why this would group to the left when everything else is right down the middle? Yes, I shot more surplus afterward to ensure that the scope wasn't knocked off. No wind either. This same ammo printed lower in the Ishi, but did not display this "left leaning" tendency.

 

[Jenrick]

Barrel twist and the rotation it gives the bullet can do odd things. A major change in velocity will cause a large change in rotational speed.

-Jenrick

 

[Abel]

^^^this^^^

 

[Sistema1927]

You are possibly on to something, but the GSR is 1:10 RH (16.25") while the Ishapore was 1:12 RH (19").

Doesn't seem to be enough to result in such a difference, but you never know.

 

[Bart B.]

You will get heavier loads impacting further to the right from lighter ones if you're right handed shooting hand held sporting rifles resting atop bags on a bench top. The rifle swings more to the right with heavier loads because it's recoil axis is aligned to the right of your body mass holding it and twisting from its recoil before the bullet leaves. My benchrest zeros are typically 1 to 2 MOA right of standing, sitting or prone zeros.

That happens to me shooting pip squeak loads with light bullets then shifting to heavy bullets loaded to the safety hilt with the rifle on bags stops bench I'm sitting to the left of.

A wide spread in twist rates will cause insignificant bullet precessional drift through 300 yards; twist differences are not the issue. You may have also mounted the rifle differently.

 

[Jenrick]

Assuming the same velocity (lets ay 2700 fps) a 1:10 gives you 194,400 RPM. A 1:12 gives you 162,000 RPM. So about a 20% change in RPM.

-Jenrick

 

[MEHavey]

Barrel whip.
Pure & simple.
https://www.youtube.com/watch?v=PEBMlfhlxYg

 

[Crowcifier666]

I've heard of barrel whip on Mini 14s, and obviously seen a bit in your video with an AR. But is a .308 bolt gun, with a really short barrel to boot, really that vulnerable to barrel whip?

I'm no expert but it sounds like Bart B. has the right idea.

 

[jmorris]

Do the test with something like this.

https://www.youtube.com/watch?v=6vpDm1bpey4

You will eliminate yourself as a variable.

 

[The Big D]

Nothing weird at all here. That's the harmonics of your barrel. You didn't think it was rigid, did you?

 

[jmr40]

It happens. I load for three different 308 rifles with bullets ranging in weight from 125-180 gr. Most of the time they impact so close at 100 yards with all weights to not matter. At longer ranges the windage is the same and drops are predictable with heavier bullets hitting slightly lower.

I really like the accuracy and speeds I get from 130 gr Barnes bullets at near 3100 fps, but they always impact 3-4" higher (not really unexpected), but 3-4 " right compared to all other loads. If I correct zero for them, all others are low left.