Bolt lapping in an Ar 10 platform?

[1Sg (ret)]

Is there a way to do this?I have a high end rifle that is shooting 2 distinct groups.Everything is strait on the gas system.It shoots a 3 rd group 1 in one hole and 2 in another an inch or so apart at 3 oclock and 9 oclock. it also does this with 5 round groups.It goes back and forth.1 round high right of the aiming point round 2 is high left of the aiming point.Then round 3 back in the first hole.Has anyone ever had this happen to them?If so what is the fix?Thanks Richard Ps all groups fired with all different brands of factory 175 match ammo.barrel with 11.25 twist

 

[Hellbender]

Re: Bolt lapping in an Ar 10 platform?

It's probably not your bolt needing lapped, it won't make up 1" of group difference.

1st, let a known good shooter shoot your rifle and see if he replicates the problem, AR10's are very hard to shoot consistantly, esp. if you are not a seasoned shooter.

Mechanically, it could be one of these issues (assuming you already have a free floated barrel tube).....

Sounds like something is possibly very slightly loose (bbl nut, forend or scope mounting) or you may have a scope issue.

Something else that sounds very likely......when you place your sandbags under the forend, do you put them out at the very tip of the forend (near the muzzle) or close to the receiver?

If you upper to lower receiver fit is a little sloppy and you put your sandbags near the receiver, it can show up as 1"+ of vertical in your groups.....

One of these may help.....
http://www.brownells.com/.aspx/pid=16662/Product/AR-15-M16-ACCU-WEDGE

But good receiver fit or a nylon tipped screw installed is the best.

 

[Ledzep]

Re: Bolt lapping in an Ar 10 platform?

This may or may not be useful, but when I shoot my 260 dpms, if I don't hold the rifle tight to my shoulder, and lock my cheek to the stock, I get about 1.5 moa vertical spread and less than .7 horizontal. Most rounds end up on the high or low end. Not many in the middle.

If i do lock down, sub moa circular groups.

 

[Hellbender]

Re: Bolt lapping in an Ar 10 platform?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Ledzep</div><div class="ubbcode-body">This may or may not be useful, but when I shoot my 260 dpms, if I don't hold the rifle tight to my shoulder, and lock my cheek to the stock, I get about 1.5 moa vertical spread and less than .7 horizontal. Most rounds end up on the high or low end. Not many in the middle.

If i do lock down, sub moa circular groups. </div></div>

Ledzep...this is very typical of a loose upper/lower fit (or can be bbl nut loose), when you are locking your cheek down hard, you are removing the up and down play in the receiver fit.

 

[BCP]

Re: Bolt lapping in an Ar 10 platform?

IIRC back and forth movement can do that too. Two distinct groups I'd think it was a shifting cheekweld/parallax.

 

[leadbullet]

Re: Bolt lapping in an Ar 10 platform?

something is loose, check barrel nut, scope mount, scope rings etc.
Hope this helps.