This has been done to death and I've done a lot of research on it, so my question is just asking about "the fix" I've employed.

Pertinent components are a KAC URX 3 rifle length rail, a VLTOR MUR 1-A upper and a LaRue Stealth 18" barrel (also used a Faxxon 18" barrel).  Regardless of which barrel I use, the end result is that it's about a millimeter to the left when looking at where the barrel exits the handguard shroud.  Yeah, I'm sure I can just adjust the rear sight for the offset and it would work fine, but I'd like to get it as centered as possible.  I tried two different barrels - both with the exact same result.  I suppose it could be a bad VLTOR MUR upper, but that's more unlikely than the URX just being a bit out of spec.  I also have two factory KAC uppers with a URX II handguard - and while mine are centered, I've heard reports of other factory ones that are a bit off like the one I'm building.


Either way, I don't like it being off, especially since the front sight it built into the rail.  So what I did, was to cut a barrel/receiver shim in half and before putting the barrel back on, I put it on the side of the upper that needed to be compensated for, and then put the KAC barrel nut back on, per torque spec's.  The barrel is now perfectly centered.  Is this an adequate fix, or am I setting myself up for some other problem later on?

It really doesn't matter but the ultimate fix is to lap the face of the upper receiver with something like this :http://www.brownells.com/gunsmith-tools-supplies/rifle-tools/receiver-tools/ar-15-m16-upper-receiver-lapping-tool-prod20220.aspx

I've seen discussions on lapping, but that's typically done when poor quality components are used in order to correct grossly out-of-spec issues and I've never seen it recommended by any well-known AR builders.  In fact, I've only seen it recommended against being done.  

Particularly, I don't think it's the barrel or the upper that's causing the issue - but rather the URX rail, so lapping would likely not have an effect in this case (just my guess).


Just wanting to get some opinions or feedback on the shim method, and if it will cause any issues when used as I described.  

Your shim did the exact same thing as that tool would do - it squared up the face of the receiver. My concern would be the shim falling out and the barrel becoming loose.

If you're familiar with the install of the URX rails, the shim cannot come out, as it's enveloped by the barrel extension on one side and the rail on the other, with the KAC barrel nut holding the barrel itself into the rail and upper.  So if that's the only concern, then it should be fine.  I didn't know if there was concern about having a small "gap" on the opposite side of where the shim was installed between the upper and the barrel (ie, pressure cavity or something like that).

I used the shim method on my last build and it worked perfectly. IMO there is no way the shim can fall out.. The shim I used was round, I cut it so it would fit around the locater pin and slide flush against the barrel flange. When the barrel nut is attached the shim is captured but the nut.

If the face of the upper is square to the centerline of the upper then by adding a partial shim you have created one of the conditions that people lap uppers to get rid of.
That is the barrel does not point to the same place the upper does.

I've always had the impression that the shim is used when you cannot get the barrel nut to time with the gas tube to receiver fit.  I'd at least check the face of the receiver for squareness.  You don't have to remove any material to check how much contact you have.

This would be correct.

The face of the upper needs lapping.  The only other reason this symptom would show up is if the threads for the barrel nut were not cut concentric to the centerline of the upper.  

Even a few thousandths of high or low spots on the face of the upper is multiplied down the lengh of the barrel.  You can't see the cant with the naked eye, and it's doubtful it could even be seen with a straight edge.  This is exactly the reason I lap all uppers I build before I assemble them.  In most cases the face of the upper is fine, but I've had a couple that clearly had high spots and would have created this exact condition had they been assembled.  An ounce of prevention (and a little bit of lapping compound and about 30 seconds with the tool) truly is worth a pound of cure.

It's rare, but not so rare that a tool wasn't deemed necessary to exist.  There is also a reason ALL benchrest guys do this to their uppers.  They want an absolute perfect delivery of clamping force to the barrel extension flange.  An uneven face brings uneven toque into the equation, and that little bit of nothing is something at a 1000 meters.

Lap the face.  I'm betting dollars to donuts that's where your issue is.