Lapua brass POI shift

[Bunnypunter]

My Google-fu weak and I could not fin info on this subect; so here it is. I finally broke down and bought some Lapua .308 brass. After loading it with my same load parameters as my other used Winchester brass, my POI had shifted about 2.5" lower at 100 yards. Horizontal did not change, just the vertical. WHY? I did a new OCW test to confirm that my charge had not changed dramitcally and found a better node 0.2gn less, but I assume this would not effect POI that dramtically. Any insight would be helpful. I would just like to understand this phenomenon a bit better. Thanks in adavance for your help.

 

[Sugarbug]

I would imagine you're dealing with pressure differences due to varying wall thicknesses between the Lapua and Winchester.

What distance was this 2.5" drop?

 

[seanhagerty]

With a given charge, brass capacity afffects pressure, velocity and POI.

 

[Dan Newberry]

yeah... what Sean said. Barrel time is the final thing in the equation. Your barrel moves up, down, left, and right as the bullet is going down the bore. The direction the muzzle is pointed when the bullet is released will therefore change, based on the barrel time. By barrel time, I mean the length of time the bullet actually spends in the barrel... so what you've seen is to be expected.

But I will say that unless you're shooting a light weight barrel, a 2+ inch point of impact shift is on the extreme side.

However... if the load is grouping well, then all is well.

Dan

 

[AK Sgt Maj]

I shoot both Winchester and Lapua brass, in .308, and use the same load, bullet, primer, powder and bullet seating depth, and my Lapua loads are about 60 FPS faster than the Winchester, and my 200 yard zero is the same for both. I would think that if they were different at all, the Lapua loads should be higher, not lower. The Lapua brass is thicker than the Winchester, is why it is a little faster with the same load.

 

[2Shots]

I would think you could take any commercial brass, win, rem, hornady, nosler, or lapua and load the same load and be within 3/4 moa. Even with different case wall thickness and similar neck tension, especially on the vertical, that relates to velocity not barrel whip so much. I'd sure load up 10 of each and run them across the chrono. It would take a pretty good velocity drop to equate to 2.5" at 100 yards. Doesn't make sense to me. As AK said, I would expect hotter and higher because of the reduced case capacity.

 

[glassbeaver]

Your difference in POI seems a little extreme, but as others have stated, with the same charge you will get more velocity out of the lapua due to case capacity. Back it off half a grain and see what happens.

 

[melloyello]

Have you shot any in the Winchester brass since you noticed this POI drop with the Lapua? If not, I would do so to make sure your zero hasn't changed for some reason.

 

[jkonzal]

I would think it's also going to be different shooting the brass after reloading it again....after it's fire formed.

 

[Bunnypunter]

It sounds like there may be other issues affecting the impact shift. I'll head back to the drawing board and verify with my other load wiht Win brass. Thanks for all your input.

 

[melloyello]

I would think it's also going to be different shooting the brass after reloading it again....after it's fire formed.

From my limited experience, full length sizing and just neck sizing causes no shift in impact if all other variables remain constant. I can FL size and shoot 2 times usually before I have to FL size again. Through all 3 firings the POI is the same.
But lately I have stuck with FL sizing everytime to keep everything as consistent as possible. It keeps me from having to mark everything to keep up with how many times it has been fired and FL sized.