Lapping uppers

I have two uppers I plan to use for builds this summer. One is a slick side from AP and the other is an unknown mfg I purchased from RTG a while back. I wanted to share results of lapping the uppers.

I can't quantify how rough the face of these uppers were but I could see a slight ridge on both the inside and outside edges where the barrel flange would mate, and I could detect some roughness by feel. So I got the PTG lapping tool and some valve lapping compound and gave it a shot. Both uppers still show some of the original finish in some places so that will tell you how little material I took off. In other places there is bright metal in a ring so clearly some metal was removed. They look and feel perfectly flat now.

The technique I used was to NOT mount it in a vice block as shown in various videos on the subject, but to just slide the uppers onto the tool and hold them in a gloved hand. I did it this way because I wanted to see if the uppers and the tool were coaxial or not, which I figured I could only tell if I held the upper. The tool fit was very tight on both uppers and I used lots of motor oil to get them in. I used a low-rpm drill and did not detect any wobble. Otherwise I followed the advice to check my progress often and apply the valve compound in such a way that it would not get into the inside of the uppers.

I am glad I did it based on the before and after feel. I will follow up with a report about how off-center the mounted barrels are by comparing a bore sight with the mounted sights, which will take some time.

Nice.

Thanks for sharing.

A brief treatise on upper lapping.
Very poor practice IMO.
Unless it is a cheap assed $25.00 upper from SARCO or the like I have yet to see it make a tangible difference. In all of the AR platform rifles I have built I can only recall 2 instances where the receiver ring was far enough out of perpendicular to the receiver centerline it would have made any difference. In both instances the uppers were replaced with known good uppers.
Any error on an upper should be easily compensated for by dialing in the optic.
As a matter of fact I feel it does more harm than good.
The only thing keeping the steel of the barrel extension from contacting and for all intent and purpose battering soft 7075 aluminum is the hard anodized finished of the receiver ring face which is removed by the lapping. Even though the barrel nut holds the barrel extension tight against the receiver nose (for now) the repeated recoil impulses can cause plastic deformation of the receiver nose resulting in a loose barrel.
That .003 of anodizing is essentially a "case hardening" for the aluminum and you just took it all away.
We also need to look at it from a Metrology standpoint which is critical when truing mechanism to concentricity, perpendicularity and axial alignment.
The lapping tools I have seen are at best a slip fit in a receiver with a +- .003 tolerance (IIRC) allowance on the bore. Impossible to obtain true perpendicularity with that kind of allowance.
The only way to to obtain an absolutely perpendicular and square ring face is to use the barrel "socket" surface of the receiver as your datum as this is the true registering surface for the barrel. One would have to have a mandrel and a series of bushings in .0005 increments to accurately "dial in" the bore centerline similar to dialing in a bolt action for truing.
Waay too much work IMO for negligible gain. I have done a few but have not seen a measurable difference.
If you want to invest some time in something that will make a known and proven improvement bed the barrel to the upper or just LocTite or Devcon the barrel to the upper.
On an accuracy rifle the barrel and upper should be treated as a disposable unit.

I don’t know, Bert. I like your thinking, but I’m not sure about a barrel extension “battering” the receiver. The two pieces are in intimate contact and under considerable compression due to the force of the barrel nut. The barrel extension doesn’t have any momentum to beat the end of the receiver because there’s no opportunity for it to move relative to it. I’ve taken apart several high mileage barrel/receivers that I’d lapped and I saw no evidence of peening.

I do it. I’m not convinced in the slightest that it provides any value, but I’m pretty certain it does no harm. To be fair, I glue my barrels in so there is even less chance for the extension to beat on the receiver.

Thanks for feedback and perspective.

My goal was two-fold. First I wanted to know that the barrel flange was seated against the flattest possible surface and when I saw the slight uneven surfaces on the face of the uppers I thought lapping would resolve that. If there is going to be some deformation from steel slamming against aluminum it seems that there will be less if the mating surfaces are as flat as I can make it, as opposed to small concentric ridges. Secondly I want to have the sights as close to centered once I sight it in because if they are off to one side I will be forever haunted by the thought that I could have lapped the upper and didn't. (This last point is probably an OCD thing)

BTW: I am planning on bedding the barrels with some loctite green goo so they will be pretty close to a single unit when all said and done.

Once I get the first one assembled and sighted in (an RTB 22lr) I will update the results - mid summer probably.

OP:
I'm all for experimenting but to be honest....... I doubt this is will show anything.
And to my concerns:
-I don't see how -by your description - you are ensuring you are at precisely a 90 degree angle, in working the upper.

Also if you want to wander into the weeds of this:
- How do you know the barrel extension ( is that what you mean when you say barrel flange?) is concentric or at a 90 degree either?
- Or for that matter the barrel is centered and square in the extension?
- Lastly how do you know the barrel retaining nut is also at a 90 degree and free of imperfections?

The three: upper/barrel with extension/ retaining nut mate together...... in order for your theory to work.... ALL would need to be checked to be parallel and at 90's.......

In other words aren't you wasting your time if you check the upper but not the other two?
Since if any of the three are off, you wont have a aligned and square mating of the three surfaces?

(You have a sandwich of upper---barrel extension and then barrel retaining nut. If any of the three are off, the mating wont be flush and true, so you need to check all three, not just one).