Squaring & Lapping the Upper Face

[Keep The Change]

I know I'm about to start some arguments concerning the effect or non-effect, and the pro's and con's of this process, but I'm considering squaring and lapping the face of my upper.

Upper: Vltor Forged
Barrel: 24" Brux Barrel
HG: Nordic Comp with aluminum barrel nut

I just got the barrel in a couple weeks ago and got a Nordic HG in this week (thanks Tedward).
I test fitted the upper last night just to check the fit of the barrel/upper and barrel nut/hg, etc.

The barrel extension wasn't as tight as I would like, it had some slop between it and the upper. I could get a 0.006" feeler gauge between the face of the upper and the flange of the barrel extension.

The Noridic HG has an aluminum barrel nut and I'm a little concerned about barrel whip coupled with the decent clearance between the extension and upper and the squareness at the extension flange and upper face. With an aluminum barrel nut holding the barrel on and somewhat loosey goosey fit, am I worried about something I shouldn't be?

I know AA beds their extensions in loctite, right? That is too combat the thing I'm worried about correct? So is my concern legitimate?
I don't really like the idea of loctiting my Brux Barrel but if my concern is an issue I guess I could.

Or would just squaring the face be enough and don't worry about clearance between the OD of Extension and ID of Upper?

Thanks,
KTC

[bigbear_98]

LRRPF52 started a thread on exactly this.

I trued the face of my receiver, shimmed the extension with a stainless steel feeler gauge blade (.002), and bedded it with loctite.

http://www.65grendel.com/forum/showthrea...Accurizing

[MikeGideon]

I have squared and bedded 3 so far, all CMT Billet uppers, and all Grendels. Unfortunately, none have made it to the range yet. The receivers were probably above average in quality, and still weren't completely square. You can tell by how the finish comes off the end of the receiver. I used the Wheeler lapping tool.

Couple of tips... Oil the bejeesus out of the pilot, and make sure none of the lapping compound makes it into the receiver. That will happen if you pull the tool out too far as you are checking progress.

[Keep The Change]

Any issues with the alignment tab on the extension bottoming out in the upper notch?

Thanks for the info guys. I'm still a little leary about bedding with loctite, but then again I plan on keeping the rifle in this config for a long time. So maybe I do it. I might end up needing pointers on that process.

[NugginFutz]

Any issues with the alignment tab on the extension bottoming out in the upper notch?

Thanks for the info guys. I'm still a little leary about bedding with loctite, but then again I plan on keeping the rifle in this config for a long time. So maybe I do it. I might end up needing pointers on that process.

No worries about the indexing pin, as the amount you remove when lapping is far less than the clearance that pin has to the face. As far as Loctite Blue bedding goes, I've only done it on one of my AR's, which had a sloppy fit (by my standards), and I intend to yank it after a month or two of test firing. I can't say how much it helped the accuracy, as I never shot it before lapping or bedding, but this 16" barrel torments 4" plates at 210 yards with an Aimpoint H2.

If you do bed the extension, just ensure you don't contaminate your threads or the inside of your extension. To remove the bedded barrel, LRRPF52 recommends soaking it with penetrating oil, and the use of a heat gun (never an open flame or torch).

[Tedward]

I have squared and bedded 3 so far, all CMT Billet uppers, and all Grendels. Unfortunately, none have made it to the range yet. The receivers were probably above average in quality, and still weren't completely square. You can tell by how the finish comes off the end of the receiver. I used the Wheeler lapping tool.

Couple of tips... Oil the bejeesus out of the pilot, and make sure none of the lapping compound makes it into the receiver. That will happen if you pull the tool out too far as you are checking progress.

I can see lapping a forged upper because it is molded/forged.

A billet upper is cut like lapping when it is made to tighter tolerances that you can do with a hand tool. Removing the anodizing is being retroactive to the finish and reducing life of the material. I have a few of the Cross Billet uppers I've been using and have had a machinest check the squareness with his gauges and they were true. He also checked a forged upper and it was not as true so that is another reason why I stopped using the forged uppers. You get a stiffer product (less flex), save time, money and the finish, just go Billet in my opinion.

I did the same for Seekins Mega and a few Mil-spec forged, all billets were true and the forged were pretty good, if you think about it, Forged are also milled when they come out of the line. Look at a raw lower, the main bearing surfaces are put on a CNC or milling machine way better than the hand lapper.....

http://firearmsdesigner.com/?p=488 http://firearmsdesigner.com/?p=594

[NugginFutz]

Not to take issue, Ted, but the uppers Mike was referring to were all Billet uppers, and yet, the receiver faces were not true. The takeaway here, I believe, is that it never hurts to check, and that having said lapping tool is more convenient that having to run down someone with CNC equipment.

As far as as CNC or milling being more accurate than hand lapping, I can only imagine that you were referring to depth control. The tool I use is manufactured by PTG, available through MidwayUSA. It produces results of high enough quality that I feel comfortable. The resulting faces are flatter than I can measure, and are always trued relative to the upper receiver axis.

Having said all that, like you, I like the billet uppers for a number of reasons, including aesthetics, and perceived rigidity. Cost is the only off-putting aspect of them.

[MikeGideon]

I don't know what depth was cut, but it wasn't much. I assume the threads are cut the old fashioned way, which could easily distort the front surface of the receiver slightly. I also have no idea what the error angle was before I started.

Honestly, I bought the tool because I was more concerned about breaking bolts than accuracy. I was assuming that the great bold famine was gonna continue. Now, I even have a hard use spare on the way from AA.

BTW, I got my two billet receiver sets in a group buy, so they were only slightly higher than forged.

[GMinor]

I almost bought that and squared the receiver.

But when I test fit my barrel the GD thing was so tight I had to use a mallet to seat the dang thing. I left it there, and that is where it will stay till it needs replaced. Shoots lights out so no complaints here.

[GMinor]

MEGA HD billet upper and PF barrel / extension

[NugginFutz]

I almost bought that and squared the receiver.

But when I test fit my barrel the GD thing was so tight I had to use a mallet to seat the dang thing. I left it there, and that is where it will stay till it needs replaced. Shoots lights out so no complaints here.

Gminor - with a setup such as yours, I would be hard pressed (no pun intended) to imagine the receiver face could cause any misalignments, since your fit is so snug. I know of at least one billet upper where the mfgr recommends freezing the barrel & extension, and heating the upper, prior to installation. Now, THAT sounds like a tight fit, to me.

[Tedward]

NF, I just haven't seen the need to lap it for that Cu-t hair of a measurement to make that much difference. They do make the tool for a reason so each to there own. If I were to use a mil-spec upper, I would possibly put the money into one.

Some people have had square issues but I really wonder if that was the actual problem or maybe when it was assembled they had it out of square or tightened incorrectly.

I just got one of these from Brownells and it makes installing the upper to barrel much easier.
http://www.brownells.com/gunsmith-tools-...27452.aspx
http://www.brownells.com/gunsmith-tools-...od468.aspx

I am also use the Mega SBU Uppers and they also are so true the only difference in measurements would be the anodizing. Older model Mega SBU Uppers needed to be heated up so the aluminum would expand to get the barrel extension into the upper. The new ones, about 8 of them so far, the barrels go right in. Mega increased the size on the upper of the extension receiver so the heating process is not needed any longer.

Lap or No Lap, I guess personal preference.

[GMinor]

Gminor - with a setup such as yours, I would be hard pressed (no pun intended) to imagine the receiver face could cause any misalignments, since your fit is so snug. I know of at least one billet upper where the mfgr recommends freezing the barrel & extension, and heating the upper, prior to installation. Now, THAT sounds like a tight fit, to me.

It came to that, but my barrel wouldn't fit in the freezer. So I went the next best route, bigger hammer and that oil you get with a geissele trigger (that stuff creeps like crazy!).

[Tedward]

I never froze a barrel but use my heat gun that is around 500 degrees but keep it moving and heat the front of the upper while its in the vise. A block of wood on the end of the barrel and tapping it was required for sure.

[NugginFutz]

NF, I just haven't seen the need to lap it for that Cu-t hair of a measurement to make that much difference. They do make the tool for a reason so each to there own. If I were to use a mil-spec upper, I would possibly put the money into one.

Some people have had square issues but I really wonder if that was the actual problem or maybe when it was assembled they had it out of square or tightened incorrectly.

I just got one of these from Brownells and it makes installing the upper to barrel much easier.
http://www.brownells.com/gunsmith-tools-...27452.aspx
http://www.brownells.com/gunsmith-tools-...od468.aspx

I am also use the Mega SBU Uppers and they also are so true the only difference in measurements would be the anodizing. Older model Mega SBU Uppers needed to be heated up so the aluminum would expand to get the barrel extension into the upper. The new ones, about 8 of them so far, the barrels go right in. Mega increased the size on the upper of the extension receiver so the heating process is not needed any longer.

Lap or No Lap, I guess personal preference.

Ted - I'd read about that barrel nut tool in a previous thread. How do you approach aligning the gas tube, when using the tool? I'm curious, since the upper is facing down.

I like the idea of avoiding applying any torque to the upper receiver, regardless.

[montana]

I bed with blue loctite after lapping the front of the barrel extension. I have removed barrels that have been bedded without any issues at all. It makes for a very tight barrel to upper receiver fit.

I have put my barrels in the freezer to aid the attachment of tight gas blocks with good results. We did this for years to instal tight cups on heavy equipment and trucks.

[explorecaves]

Ted - I'd read about that barrel nut tool in a previous thread. How do you approach aligning the gas tube, when using the tool? I'm curious, since the upper is facing down.

I like the idea of avoiding applying any torque to the upper receiver, regardless.

You can mount a socket breaker bar in the vice and then have the extension tool just outside the vice pointed up. Voila! The upper is now facing up so you can see the gas tube alignment.

[customcutter]

KTC,

I checked my CMT billet upper and it is square. However, I had a slip fit, but no "wobble". The barrel extension IIRC was .9965. I decided to put mine together without loctite for now. I wish it had been a tighter fit, but I may do a before and after comparison if accuracy is suffering.

The main reason for doing it is to make sure that the face is square to the bore. Like others have said, it may affect accuracy and also help address the issues with broken bolts. Once I get some "used" brass, I plan on lapping my bolt also to make sure it is square, and has even contact.

Hopefully a CNC machine would have tighter tolerances, however machines tolerances deteriorate and wear, and if the machine has been "crashed" (hit something it shouldn't have been cutting or bumped into something) then the indexing on those 4 or 5 axis's could be off. I don't know how often the heads are indexed, but I'm pretty sure it's not every day at the start of the shift.

If you decide to loctite, as was already said, no open flames (to hot for the heat treat on Al), use a heat gun, heat evenly and penetrating oil. Loctite the barrel extension, not the threads.

[Tedward]

CC, As for my machine shop, my guy has 1970 old rock solid American made machines and are more true than some of todays products so I agree with that. It is a machine by machine advantage or disadvantage. His are milling machines so many small 4 &5 Axis CNC machines might have lower tolerance. Some high end EXPENSIVE CNC Machines probably have better tolerances.

My CMT's are a great fit too and no wobble. I guess it depends whos making the parts and if you use parts from 4 different manufactures, you could get differnt tolarances so like your saying, just need to check them.

[Tedward]

Ted - I'd read about that barrel nut tool in a previous thread. How do you approach aligning the gas tube, when using the tool? I'm curious, since the upper is facing down.

I like the idea of avoiding applying any torque to the upper receiver, regardless.

I can look right down thru the upper charging handle space see the hole next to the shaft, I can see the floor. Then once its at 30ft lbs, it is close, I insert an alignment rod, gas tube, and turn slowly till it falls thru the hole. Gravity.

Before I used a rubber band and a short piece of picatinny I mounted to the top of the upper. When it lines up, the gas tube was pulled into the hole. The new method is much easier. I put 10 barrels on CMT uppers in an hour yesterday.