My son's AR10 in 308 consistently hits ~2MOA left for the first shot compared to the remaining rounds.
I assume this is because of hand charging the first round vs auto loading of the other rounds.
The gun consistently shoots sub MOA groups if you exclude the first round. The first rounds get a nice group also just a couple of MOA to the left of the main group.

Is there a know cause & fix for this?

New Criterion barrel in an Aero upper/lower.

Did you lap and square up the upper receiver to the barrel?  Loctite bedding?

What ammo or loads?

Open bolt load. Try again.

First round of every magazine?

Or first round of the string.

Cold bore shift is real on some barrels. Then again so is the psychological effect of the first round recoil changing your follow up trigger squeezes

Hand chamber the first round and see if it helps

Hand chamber every round and see if they group

This is sometimes due to the gas tube being out of position and/or to long. Something to check for anyway.

Thanks for the replies all.
To answer the questions:

Did you lap and square up the upper receiver to the barrel?  Loctite bedding?
No but barrel extension was a tight fit.

What ammo or loads?
175gr sierra matchking over varget. This is a very accurate load in both a Ruger precision & the AR10 (minus the first shot)

Open bolt load. Try again.
??????

First round of every magazine?
Yes

Cold bore shift is real on some barrels. Then again so is the psychological effect of the first round recoil changing your follow up trigger squeezes

My son is a very good long range shot. Consistently gets very tight groups with his Ruger precision in 308 & 338 Lapaua.
No first round flinch and this is not a first shot cold bore, it is a first shot from each magazine loading.

Hand chamber the first round and see if it helps
That is what he does now using the charging handle.

This is sometimes due to the gas tube being out of position and/or to long. Something to check for anyway.
Thanks, I'll have him check this.


To clarify the problem.
AR10 shot from the magazine. First round from each magazine hits ~2-3 MOA to the left. Shoot several magazines in succession and he gets two groups. One to the left `2-3 MOA and another at point of aim. The first round from the magazine shift is very consistent going to the left.
Not a cold bore shot or loose barrel issue since only the first round from each mag load goes left from the main group.
The main group is sub MOA - a very accurate load & gun excluding the first shot from each mag.

This problem has both of us baffled.

I think "open bolt load" means single loading.

If it's not the shooter position settling in after the first shot, I'm thinking carrier fitment in receiver being affected by follower spring pressure, though 2-3 MOA seems extreme for that.

Have y'all carefully ejected a few rounds to see if the first has a wicked scratch? Does the mag fit loosely fore-aft in the well - I wonder if he holds the magazine when pressing the bolt release, making the first round be more nose down than the others.

Shoot the first round over the target into the backstop....

problem solved.

Is the gun broken in? Mine cleared up by a few hundred rounds.

My buddy has a similar problem with the same components, he usues his for hunting so he lives with it. His isnt 2 moa off from the grouping, its more like a minute. His is 2 rounds hight left almost all of the time.

This happened to me on my AR-10 build.  I would get these really bizzare groups I had never seen on a gun before.  After sitting down and reallly thinking about the beahvior of the groups; I broke the front end of the gun down and cycled the action slowly.  Sure enough the gas tube was impinging on the bottom of the key enough that you could see the gas tube flex slightly.   Ordered a different gas tube, installed it and the problem disappeared.

Look at this wild stuff.  I went from this:





To this:




I am not saying that this is the problem with the gun in question but it is definitely one of the things to look for based on the symptoms exhibited.