Thread: Unexpected POI shift

At the range I had an unexpected POI shift between two loads.

Ruger American .308, Sonic suppressor, sighted in at 100m with 150gr Nosler BT@ 2,650 fps. Then shot 130gr TTSX Barnes @ 2,850 fps - POI 2" lower???

I let the barrel cool and repeated with the same result. I was expecting the Barnes to impact same or slightly higher.

did you cheek with the 150bt to see if they still hit the same poi?

Originally Posted by Crusty

At the range I had an unexpected POI shift between two loads.

Ruger American .308, Sonic suppressor, sighted in at 100m with 150gr Nosler BT@ 2,650 fps. Then shot 130gr TTSX Barnes @ 2,850 fps - POI 2" lower???

I let the barrel cool and repeated with the same result. I was expecting the Barnes to impact same or slightly higher.

If you have pics to post it can be helpful?

yes, ran the 150BT, then the ttsx, cooled then repeated 150bt followed by 130ttsx

You'd expect a POI difference and there's no reason to expect that it should be in any particular direction


The 150s probably recoil more, moving your POI up the page

No reason to expect the same POI with very different pills.

Originally Posted by chainsaw

No reason to expect the same POI with very different pills.

Agreed, but two inches at 100m? I have never seen that much difference - an inch maybe.

Yeah, I agree with previous comments. Always sight in with a particular load, then only use that load for the next hunt. You can take note where the groups shot relative to each other with the different ammo, then adjust your clicks on your scope for the particular load you are using that day. The problem is keeping track of which load you adjusted your scope for. On my 223 with a digital night vision scope, I have different zero profiles loaded for about 4 different types of ammo loads, as they all shoot different slightly different POIs. Just have to make sure you are using the right zero for the right ammo and you are away.

You would just be lucky if two loads happen to have the same POI, it mostly doesn't happen though. Especially with different weights and velocities.

Originally Posted by gimp

The 150s probably recoil more, moving your POI up the page

only .2 ft-lbf difference in recoil between the two loads - I doubt that would make much difference.

I have checked scope mounts, action screws. Was shot off a front rest.

2" difference at 100 doesn't surprise me at all. The pills are different weights and different velocities, one is a mono, one is lead. Its totally normal to expect a POI shift of this magnitude. I shoot the same Barnes and a Nosler Accubond out of my 300wsm and the POI difference would be about 2". Just adjust my scope depending on what load I am running.

It is due to the increased muzzle climb before bullet exit with the heavier bullet, often made worse by just resting the fore-end rather than holding it firmly with your hand.

Heres a pic from last time I was at the range. 3 shot group above the target was with 168gr Nosler Accubond, I then adjusted down 7 clicks and shot the bullseye. So rifle was then zeroed for that load. Then tried two shots with 130gr Barnes TTSX aiming at the square on the lower right. You can see they are shooting about 1.5-2” lower than the Noslers.

Greetings,
From my experience the .308 does move around a bit with different projectiles and velocities which can be different between rifles. It is what it is.
GPM.

Longer "barrel dwell" with the heavier projectile changes the harmonic vibration speed of the barrel

@Rushy isn't this I "thing" used by handgun users??? Heavier load,higher recoil,higher impact???? And the muzzle brake type thing helps to dampen it down. Years back I fired ten different loads into same target through my 270w they made a playing card sized group so about 2"wide and 3"high...not enough to worry about at bush hunting ranges but enough to matter out further....so I keep a few loads that I know are zeroed in handy but not always up the spout so to speak.