Lapped 2 mega uppers & 4 aero’s But the compound worked its way down shaft as I lapped reciever. When finished I took a light and looked inside reciever,the anodizing was scuffed but not close to being removed. Am I fooked or am I just being paranoid?
I used a 180 grit compound. Just wondering if this can create a problem maybe. Thanks.

You are just being paranoid, I have raw uppers that I have been shooting for several years now and no problems at all..

You didn’t damage anything.  But why were you lapping with abrasive.

Most people seek a tighter fit to the barrel extension, not a looser one.  If anything, you want to true the face of the receiver.  Although without something to index from, that may be a boondoggle as well.

He was using a lapping tool to lap the face of the receiver and accidentally got some lapping compound inside the receiver.

This.  He just put too much lapping compound on the tool face.

I've always been skeptical about the likelihood of making a receiver face more square to the bore centerline than what comes from the factory by removing material with a hand drill, a metal rod, and a bench vice. The whole idea that removing anodizing tells you anything about squareness is questionable. After all, if you start with a square face but don't run the drill perfectly square, you will  remove anodizing unevenly even though you're making things  less  square, not more. At these tolerance  levels, the free play necessary to allow the tool to spin in the receiver and variability in the tool is enough to take  perfectly square out of the equation. Even then, if the innards of the receiver aren't square to the imaginary bore, the tool won't be  either. If the receiver face isn't square, why would anything else in the receiver be any more  square?

I can see lapping  receivers that don't mesh well with an extension. However, lapping every receiver every time  seems like a  tail chasing exercise to  me. There doesn't seem to be any guarantee of improving things, and there's a  chance of making them worse.

The lapping on the Mega Arms billets was unnecessary as it comes already in spec and evenly finished at front edge.

The aero has a tighter bore inner diam that requires often some light rubber mallet work to fully seat (or first heated upper/chilled barrel and then mallet) and thus makes the deviation from the front edge insignificant, especially when barrel nut torque is much less than max of 80 ft/lbs.

The major help to truing an upper would be if there's slop between bore of threaded area and barrel extension when the front edge is machined off a bit or has extra anno.

When I have lapped, I use BreakFree CLP on the tool except for lapping face, which has small amounts of compound applied at a time, making sure to remove excess/old after a 5 seconds or so before replacing. Midway, I will check and use break cleaner to remove any starting to travel in the bore. I also use a clamshell to hold the upper vertically to apply even force by tool. As I usually use good parts, I have only needed to lap Anderson uppers.

At this point with damage, one might send those to a machine shop for lathe work, which can be cheaper for a couple of uppers versus same cost buying a tool, compound, etc. Then bed if you plan on not removing the barrel.

Imo, I would just sell uppers on EE with full disclaimer and use funds to start with fresh, quality parts used as is, ie the same parts/manufactures bought originally.

The innards are the bore, so they kind of have to be square.

I think the idea of lapping the face of the upper may be more important when the assembled upper can't be sighted properly with iron sights, or where the barrel ends up not centered within a properly installed rail.   I've had uppers where the rear sight was all the way over to the left or right in order to be sighted in.  Lapping the upper face has helped enormously in that situation and allowed the rifle to be sighted in with the rear sight at or very near center, and also helps center a barrel inside a free float rail.

This^^^.  That Brownells tool is snake oil bullshit.  If you really want to square a receiver, you need to properly index it off of a control surface.  Putting a damn slip fit tool in a hand drill and "lapping" the face is not going to make your receiver better than it already is.

I thought the value wasn't squaring it to anything, but rather providing a lapped surface for the barrel extension to mate with. Similar to having super tight (heat required) fit for the barrel to into, this makes your barrel and receiver more like a single part....

and you're wrong

the sad part is you probably don't know why

Sadder yet is when someone stoops to condescend rather than take a moment to explain the reason why someone else may be wrong.

Not saying I agree or disagree, because you didn't even bother to explain.  I thought this board was to share opinions and factual information to improve the knowledge base for everyone.

I'd love to know your reasoning as to why he is wrong, and perhaps others here might as well.

I've always been skeptical of those tools too and personaly would not encourage anyone to use one. As was said, the tool has to fit loose in the bore to allow you to insert and spin it, so nothing is really keeping it true. IMHO, you stand a good chance of taking a square reciever face and making it not square anymore because you lapped it with a loose-fitting tool that you could be holding crooked.
Same reason I wouldn't want to use a drillpress that wobbled.

I would like to learn more about this.

When a rifles requires the rear sight to be near all the way left or right to zero, then you run the lapping tool and get it zeroed with the rear sight near center, it's doing something good.  It's definitely not making it worse.  Of course it's not as precise as a lathe, but it's more precise than most forgings.

I don’t claim to be an expert, but I am a machinist/ tool maker and have a side business building long range rifles. There are some on this board that see more rifles in a year than I will in my lifetime, but I probably work on more than most on this board.

There are several thousandths clearance between a lapping bar and the upper. Because the guide is so much longer than the truing surface, that translates to a few tenths minimum to a couple thousandths max that the lapping face can be misaligned. I can measure and do the math if anyone is curious. That’s MUCH more square  than most receiver faces I’ve encountered. Some brands are consistently pretty close, but most aren’t.

As far as a lapping bar not being as true as a lathe-cut surface, think about how the upper would be indicated in the lathe. Either a bar would be run through the upper and put between centers, or the upper would be mounted in a double cat head and a bar inserted and indicated in multiple locations. Either way depends on a bar inside the upper, with the exact same theoretical problems as a lapping bar.

Lapping bars do help square up receiver faces, and I don’t think you could make it worse with one if you tried. You can feel a difference when you tighten the barrel nut, it’s less ‘squishy.’ How much difference that makes on target I don’t know, but every gas gun that leaves with my logo engraved on it has been lapped. In my opinion, it’s a good thing.

Add another shooter here who has removed lots of windage by lapping a receiver face. I trust these little tools.

If you were to try one sometime, you would realize they fit very tight and there is absolutely no wobble.  I've corrected misalignments with mine.  The barrel wasnt even close to centered in the free float rail initially.   After lapping it was right on and the rifle shoots lights out.

because it would be wasted time on someone like him, actually most on this board. His stupid reply shows that he has never used one, or how it could be beneficial.

Its also not my duty to tell you how everything works and all the tricks to making the AR great. You can just stop being a needy 13'r and do a little research on your own. Needy people and "I dont know it all" have driven off most of the knowledge on this site with these attitudes.

there was a huge purse swinging thread on this subject not a couple months back
buy a membership and search for it.

ms556 this is your answer

Why share anything or post anything in tech if most people on this board are too stupid to digest the information you would bequeath on us? If thats your opinion of tech keep your fingers up your nose and off the keyboard. There are plenty of people quietly reading who might be able to take in some good info if it was shared, but they won't find it with posts like yours.

MS556 has shared tons of constructive info over the years and I appreciate his presence in tech.

my Brownells AR-15 lapping tool has an alignment journal shaft length of 5.5" (bearing surface length).  the outside edge of the lapping surface is .60" from centerline.  from feel, I'd say the ID/OD clearance of the tool inserted into the upper receiver is not more than 3 thousandths, personally, I think it's tighter than a car engine crankshaft clearance, as a comparison, having touched both.  But I'm not going to bother putting a dial indicator on it because I don't need to convince myself of anything.

if you do the math, the 3 thousandths of longitudinal "wobble" will translate to 3 ten-thousandths of flatness error at the lapping surface, max (ratio of 5.5 to .60, multiplied against .003").  that's pretty damn good.  I'll take that as a precision surface for clamping my barrel extension flange to it, as opposed to an as-bought surface, in order for the barrel to be true to the receiver bore, and for the muzzle end to be centered in the rail.  If you're using drop-in USGI plastic handguards you'll never notice the difference, until you have to crank the windage all the way to one side or the other on your iron sights.

just trying to eliminate the variances and contributors to tolerance stack.  now it's down to the concentricity of the barrel bore in the barrel, etc, and that's probably got something to do with barrel manufacturer quality, or maybe not, if that's your belief.  FWIW.

lapping the receiver has nothing to do with whether the gun goes bang, or click, or jams.  does not improve mechanical reliability one bit.  I guess that makes it Snake Oil, then.  I'll stop swinging my purse now.

It does though.  The truer the face is to the bore the more evenly stress will be applied across the bolt lugs, increasing the bolt's longevity.

Thank you, sir.  I too, believe in the benefits.  I have an automotive machinist background and am accustomed to working in "tenths" (.0001") increments.  My math came out close to the post by rpoL98 above, more like a maximum of four thousandths with the Wheeler mandrel on the "sloppiest" upper I've done and more typically half that.  That is still only about 3 tenths at the face.  Miniscule.  Automotive crankshaft journal tolerances.  We grind to plus or minus one tenth or two tenths, depending on the application. In addition to better aligning the rear sight to the rifle bore,  the barrel extension gets a very flat and virtually square upper receiver face for the flange to seat against.

Honestly, I only do this to center the rear sight, if it is far off on the original assembly, or when seating a match grade barrel.

I was just frustrated with the post above as not being consistent with the values we should expect from this board.  Even when someone is wrong, and who hasn't been,  we ought to take a moment to explain, and not just blow someone off. It takes but a moment and helps everyone.

What type and grit lapping compound are you using to lap the receiver face?

I have wheeler 320-grit.  Works quite nicely; doesn't take much away so it makes it easy to go slowly.