extremely consistent shift in group impact point just with different brass ?

[doubleDarms]

I ran into something the other day that kinda took me by surprise, am I was just wondering if any others have had similar results. I've been shooting some 60 gr Hornady V-max's out of an AR-15. I had them sighted in about 2" high @ 100 yds. I'd toyed around with a few things and just wanted to double check my sights before a coyote hunt. I fired a group and they were 3" high and an inch to the right. The brass was LC, the original sighting in was with FNM. All other components and procedures were exactly the same.

I re-fired this test twice more and got the exact same results. FNM 2" high centered on bullseye, LC about 3" high an an inch to right. The consistency of the LC brass shooting to exact same spot is kinda what threw me. I know any change in components, primers, powder lot, whatever will do this. But I'd never experienced it with just brass. Is this just the difference in case volume showing up ?

 

[TonyR]

I weighed 500 6.5X284 Lapua cases and ended up with an extreme spread of 6 grains in the weights. I loaded the extreme cases with otherwise identical loads and the extreme velocity spreads were about 70fps which is the equivalent of nearly a grain of powder with that chambering. Regardless of the distance, 70 fps moves the POI a bunch on the paper. Sorted within a couple of tenths, these cases shoot excellent vertical out to 1000 yards.

 

[Johnboy]

I to agree with the others volume can and will do this.Lc brass is alot thicker than over the counter brass.for me I was able to drop 2 grains to get the same loads with LC brass.so be carefull when loading with LC brass.might just get a bigger bag for the buck than you wanted.

 

[BoydAllen]

I would be interested in how the loads chronograph. Also, it would be interesting to see what would happen if you load the different kinds of brass so that they produce the same velocity.