Is this really something that is worth doing in a precision type of build? I have built about a dozen uppers over time, and a few of them in precision applications that shoot .75 moa 5 shot groups with handloads, not tooting my own horn as I know there are far more accurate examples than that but I do have some experience with what goes into an upper that shoots well.

I am doing a 6.5 Grendel build for myself using a Faxon match barrel in a VLTOR MUR. I do not consider this barrel in the same league as say a Krieger or Shilen, but would like to squeeze every bit out of it that I can. I will be using this for long range competition shooting out to 800 yards as well. I have spent a good chunk on this build and am having a hard time finding scientific testing if the process of using Blue Loctite in a barrel extension truly affects accuracy across the board. I will not be truing up the receiver face as I feel without precision tools and a lathe it is hard to be certain that the receiver face is 100% true and square with the bore after the lapping process.

1/2 MOA might be a little too hopeful but it is what I'm shooting for. Any further than that is great but for just shooting local competition I don't feel the need to pursue tighter grouping than that. Like I said bedding is not a process I have done in the past, hence why I'm curious about it. I appreciate any other methods of improving accuacy as well if someone has suggestions. I am using an Aero lower to keep tension between the receiver sets. I also always "handfit" my gas tubes and gas blocks, and using an adjustable gas block. Muzzle device is an Area 419 Hellfire that has a self-timing feature.

Edit: I will be using handloads primarily just as a heads up.

There is a recent thread debating the value and precision of lapping the receiver face.  I think it works; I’ve done it when the barrel extension slips easily into the receiver and there is any rocking of the barrel.  Lapping the receiver seems to create a flat surface for the barrel extension to seat securely to the face of the receiver.  I think the premise here is that the barrel can shift when it heats up if this is not so.

Walt Kuleck, in his AR-15 assembly guide, recommends bedding the barrel (extension) to the receiver with red Loctite to further prevent movement of the barrel when it heats up.  There are other types of loctite for high temp that may be even more effective here.  Be sure not to allow any kind of loctite on the threads of the receiver or barrel nut if you ever expect to remove it.

My personal feeling is that lapping the receiver face and loctite bedding the extension into the receiver body cannot hurt.  I do it when the barrel slips in without a rubber mallet.  Some believe if the barrel slips right in something is out of tolerance; I say nonsense, most of them are that way.

As to whether a Faxon barrel is capable of 1/2 moa accuracy I cannot say.  (Mostly I’m not capable of 1/2 moa accuracy, so I don’t know if my barrels are!)

Some barrel makers guarantee sub moa accuracy with match grade ammo; Ballistic Advantage, Fulton Armory maybe, and several of the high end deals.

Good luck with yours and happy shooting!

link to lapped receiver thread

Thank you for the info. It'd be a pain in the butt to do but would be interesting if someone did a large test of the benefits of bedding and lapping. I think they do help, but the few before and after results I've heard are just all over the place in terms of the variance of their experiences.

I am not sure if this barrel is capable of 1/2 moa either. Truth be told if it was a consistent 5 shot groups 1 moa gun I would be satisfied.

The face of the upper needs to be square to the centerline of the upper.
I have heard various numbers as to what percentage of the BE lip should be in contact with the face.
Unless a BE/upper has an interference fit I would Red Loctite bed it for your stated uses.

Get some stainless steel shim stock. Cut cut out a small strip to wrap around the barrel extension as you are fitting it into the upper. If it wont go in, trim some of the ends until it will fit with some force. I've used a mallet to bang it on to some builds. Finish up with some wicking locktite inside of the receiver (so it wicks down into the space between the extension/shim and interior receiver wall.

Better option is to have someone use a BAT Machine oversized extension and fit one to your particular receiver. Then have the smith use that extension when they chamber the barrel.

B

My most recent build:

I applied hi-spot dye to the receiver face, slid the barrel into place and applied medium torque to the barrel nut. Then disassembled and inspected. The shoulder of the barrel extension only made contact about 65-70% of the way around.

This was done as a baseline.

I turned the receiver on a lathe, on a mandrel I made. It required removal of 0.003" to square it up.

Repeating the dye test showed full contact.

As with all my precision builds I applied a wax release to the barrel extension and bedded to the receiver with JB Weld. Yes, I'm that crazy. Contrary to what you might think the peices are easily separated at the end of barrel life using a stick of pvc and a mallet.

I go through this process because I'm not a bench benchrest shooter. I shoot 3-position with sling. I want the union of the barrel and reciever as solid as possible because my type of shooting varies the forces on the rifle from position to position.

I believe the results are well worth the effort.
This particular build shoots 0.6moa off a bench with a 4x scope at a 200yrd target. (Bench only used for mechanical accuracy verification). This is a 20" 5.56 and testing was with 77gr smk.

ETA: More important than the bench test, the rifle has held zero under all conditions rain, shine, freezing, boiling hot, and all positions (offhand, sitting, prone). I put 1k rounds on it in competition this year.

Out of curiosity I got ahold of Vltor today via email and this is what Nick had to say regarding lapping and bedding. Keep in mind this is regarding the MUR uppers specifically. Figured I'd share this input.

"Due to the way we manufacture our MURs, the front face of the receiver is already very true and squared up. You generally won’t gain anything by lapping the front face on one of our receivers. We take special considerations for that from the production side."

Below is regarding bedding.

"That will also likely not be necessary (but could definitely be done at your discretion if you have the desire to do so). We control the barrel extension bore on our MUR uppers quite tightly, in regards to allowable tolerance. However, there is always minor variation in production parts, meaning on our receivers AND the barrel extension on your Faxon. It’s quite likely that the fit will be sufficiently snug, but it’s possible that you may get a high/low tolerance stacking situation that leaves you with more play than you like. If that’s the case, you could explore options like the blue Loctite bedding. I generally don’t do it on any of my builds, but I know a couple people, who I consider quite knowledgeable, that do."

For a long time I was on the fence for even lapping the upper receiver face on my builds until about a year or so ago when I built my first 300BLK upper. The barrel extension to upper fit (both brand spanking new mind you) were looser than any other build I had done to date, and to my mind attributed to what I was experiencing with this particular build. Noticeable inconsistent grouping and POA/POI variances  as the barrel heated up. So...  Said what the hell it was worth a shot and plunked down the money for a lapping tool and some green (Sleeve Retainer) 640 Loktite.

The upper face was indeed not totally perpendicular to the receiver bore, but didn't take much to lap true. Cleaned lapping compound off the receiver thoroughly and smeared some 640 to the barrel extension ensuring none got onto the barrel nut or upper threads and reassembled as I normally build my uppers.

Results? Way more consistent groupings.. enough so, that I went through a few other of my builds to give them the same treatment, and use it on ALL my new upper builds.

My .02 worth.... YMMV

I bed all mine.  My mur upper barrel fit was not tight.  I use 620 loctite.  1/2 moa with the Grendel is a struggle.  Handloads maybe, but the caliber is finicky to small groups especially with factory ammo.  Can it be done yes, but it's not typical.

Thank you for the input from your own experience. It would be great if it were a true 1/2 moa gun, however I feel it will not be the case. We will see how far I get with handloads!

Indian Head Gasket Shellac works great for me.

Don't just bed it lap it as well.
Since I don't use super high end uppers I bought one of these for peace of mind.They are cheap enough, no reason to not have one
https://www.brownells.com/gunsmith-tools-supplies/rifle-tools/receiver-tools/ar-15-m16-upper-receiver-lapping-tool-prod20220.aspx

I lap every build using the Brownells tool (very easy), and will bed my current precision build using red locktite or red locksett.  I don't have any studies to support any of this but it can't hurt and makes happy my OCD.

Joe Carlos on barrel extensions.

Vltor has a good reputation; however any company can slip up.
I check everything I can on my builds.
A person can use the lapping tool with dykem instead of compound to see if there is a problem.

I'm going to check out my stuff sometime this week and probably go from there.

The MURs I’ve checked have always been straight. If the barrel slides in without persuasion, I use Loctite 609.

Good to know. I was able to check my fitment tonight and it is a very snug fit inside of the receiver. Probably just going to leave it alone and see what she does.

I just had to use heat on the upper to slide my barrel in.
Aero M4E1 threaded upper / BA Premium series barrel.

I will need to disagree with one of the posters above......1/2 MOA with a Grendel is not a struggle. Every single Grendel barrel that I have chambered has shot well under .500" at 100 yards with factory ammunition. I have chambered many and know what it takes to make them shoot well.

I square all my upper receiver faces in the lathe and bed all of my barrel extensions to the receiver. Does it always help? I cannot say for certainty that it does, but I know 100% that it doesn't have any negative effect.... so I do it.

First build I did the windage had to go pretty far to one side on the back sight. Later had a early barrel problem and when I reinstalled the new barrel, I lapped the upper and bedded the new barrel with Blue Loctite. Sight returned to center and the gun shoots really great. So now I do it on all my builds. Why not if you have tool and 15 minutes to do it right?

I prefer bcm upper recievers since the barrel extensions fit so tight. No need to lap or bed them.