What is going on with my .223 loads?

[sammy]

Hello fellow handloaders.

A buddy recently got a 18" Stag 3 gun 1/8 compensated complete upper. It is very accurate but is curiously ammo sensitive in point of impact.

We sighted it in, all at 100 yards, using this load:
60g. Hornday V-max flat base 26.3g. Varget LC brass CCI 41.

Then he switched to:

55g. Hornday fmj boat tail, 26.6 grains Varget LC brass CCI 41.

So after zeroing with the 60 grain load (1-1/2" group) the 55 grain bullet groups about the same but over 16" higher! Why would there be such a large difference? My 16" & 20" uppers shoot them with the same point of impact. Any ideas?

[ExtremeX]

That doesn’t sound right at all… at least not 16” worth…

Any chance the scope mount or something mechanical shifted? Did you try shooting the original load to see if the POI went back to where you started?

[sammy]

Nothing has shifted. The groups are tight and consistent.

[C3nt3rMa55]

At 100yds bullet drop is about 2" for the 55gr at 2900 fps(xm193 spec) but if the 55gr is a couple hundred fps faster there will be less drop before the bullet strikes the target that has been compensated for when he zero'd it with the 62gr load.

Regardless not factoring in sight height you shoud only have to hold over 2" so the velocity diffrence if there is one wouldn't make a difference of any more than 2" unless the bullet grew wings

[ExtremeX]

Did you try shooting the original load to see if the POI went back to where you started?

So did you try this?

[GeoffLinder]

16" difference is huge. Wolff 62 factory hits a 10" steel plate at 200 yards just a tad low compared to the 55gr I am zeroed for. This is indeed strange, I would expect a POI change from 55gr to 62gr but only a few inches.

[scotty99]

I'm having a similar experience with a new 7mm-08. 120gr vs 140gr at similar velocities both group at about 1", but the 120s are 10" high and 3" left at 100 yards. Doesn't seem possible but I've watched it happen repeatedly. When it first came up, I posted on another forum and it was suggested it might be due to barrel harmonics (the gun does have a very light barrel).

Is the gun this is happening to also equipped with a thin, light barrel?

[sammy]

So did you try this?

I did twice and results were the same.

[sammy]

I'm having a similar experience with a new 7mm-08. 120gr vs 140gr at similar velocities both group at about 1", but the 120s are 10" high and 3" left at 100 yards. Doesn't seem possible but I've watched it happen repeatedly. When it first came up, I posted on another forum and it was suggested it might be due to barrel harmonics (the gun does have a very light barrel).

Is the gun this is happening to also equipped with a thin, light barrel?

I am not sure if the barrel is considered heavy but definitely not a pencil barrel.


So I called Stag today. The tech support guy was very disinterested and no help. Said he could not help me because I am using handloads. Could not wait to get me off the phone. Guess they are swamped about now.

[Whiterabbit]

if you think it's barrel whip, you can test it easily. Make a radical change to powder charge or OAL, something to change the barrel time. Goal will be to gt the bullet passing through the muzzle at a different time, so a different place in the "whip".

See if that changes the POI.

Another way potentially to test that is to do something to the barrel instead of the load to affect harmonics. Heavy weight, one of those weird rubber things (do they even work?), anything you can that you KNOW will change barrel harmonics. Will either change things or not. partitions barrel harmonics.