I want to lap the face of my receiver so I bought the Brownell's lapping tool. I see that the recommended lapping compound for this job is aluminum oxide.

Has anyone ever successfully used the garnet lapping compound for this? I ask because the anodizing on the aluminum upper is made of aluminum oxide - the aluminum oxide anodizing is actually harder than garnet is.

I suppose that it could work since the anodizing is so thin, but I wanted to ask if anyone here has ever done it before. I was looking to buy 600 grit garnet lapping compound since I can't find any aluminum oxide compound.

No. But any compound is fine. Quick in the 300 range and a bit slower in 600. "Polishing" not necessary.

Yes, I have the Brownells version with the garnet compound, and the professional machined tool version, that came with no compound.  I used the Brownells on a .308 and it took a few times to get it lapped to a point where it was even, but it worked.  The second one I used on a 5.56 and I used the valve grinding compound that I use on engine valves.  It also worked just fine.

I have the kit.  It isn't hard, you just have to break the shell on the AL and you don't want to remove much material.  Ideally, you will get a silver ring on about 40% or so of the surface.  That's close enough.  You aren't trying to machine it off...just break the surface on the high spot.  

Be SURE to lube the interior of the upper and the tool that fits in the bore.  You can lap that as well, if you fail to lube it...

Contact Paladin Machine Shop Service, tp555(at)windstream.net

Have it lathe turned and know that it is actually trued.

Thank you for all the replies. It's not really necessary to do this on my gun (the rear sight zeros okay as assembled - only has to be moved slightly to the right), but if I'm building a rifle I feel that I might as well have the receiver face lapped so that the barrel zeros as close to center as possible.

Another thing I just realized - If I lap off the aluminum oxide anodizing from the front of the receiver, this will mean that bare aluminum is in direct contact with the steel barrel extension. Can this not cause an issue with galvanic corrosion? This is the same concept as how a sacrificial zinc anode works to prevent corrosion of a steel part on an outboard engine on a boat.

Normally there is a thin layer of anodizing separating the aluminum from the steel - this prevents the issue because aluminum oxide is an insulator. I don't want to potentially disassemble the rifle in 10 years only to find that the bare aluminum is now corroded.

Not even arguable. One of the easiest things to do, period.

In theory, yes.  


In practice, unless you are in salt water...no.


I suspect a bit of grease ends up on that surface when you assemble it as well...so that will help with corrosion.  

Yep - probably take a long time even in salt water. Not much of anything is getting between the receiver and barrel extension.  Not something I'd worry about in the least.  If you think it's gonna bug you take a page from some of the AR accuracy guys and  use blue Loctite on the extension.  Disassembly may mean you have to use a little heat and/or give the extension a whack with a broom handle, but it'll seal things up.

You're over thinking it and there's zero reason to use anything finer than 120 or 220 grit.  

120 grit is equivalent to a 42 micro inch surface finish.  220 is equivalent to 15.  I doubt there's a single machined surface on the receiver that is spec'd at anything below the standard 62 Ra.

Our Loc-Tite Rep put on a seminar recently of Loc-Tite products for our shop. He specifically recommended using purple or blue thread locker on bare metal mating surfaces, especially on aluminum if rust or corrosion is a concern. He told us that grease eventually dries out and can crack, leaving mating surfaces unprotected.

We experience issues with galvanic corrosion on the axles of some vehicles that have steel directly contacting aluminum. Grease wasn't working for us in this situation. It would be a lot better if auto makers would follow this advise from the beginning.

I've used Brownells 600 aluminum oxide grit lapping compound with the Brownells receiver facing tool on my A2 build as the barrel nut would not time under 75 ft lbs. It works well. That particular rifle needed maybe two clicks to the left to zero. I'm not sure about the garnet compound but would recommend the aluminum oxide as it does work well.

I found the easiest way to do it was to remove the port door and place the upper in one of the clam shell type upper receiver blocks. Doesn't need to be clamped too tightly. I lubed the shaft of the tool with CLP and placed it in the upper and put the lapping compound on after. Turn it by hand until the compound is black, wipe it off and re-apply. I would usually do two applications before test fitting the barrel. With the port door removed the upper could be left in the block and the tool removed, everything cleaned up, and then the plastic piece that goes inside the upper insert. Tighten the vice and spin the barrel on. End result for me was the barrel timed at 60ft lbs and windage is almost mechanically centered after being zeroed. Hard to beat that.

I was able to find 240 grit aluminum oxide at Jantz Supply. It’s a water based compound.

Edited to say this product is hard to use because it doesn’t want to stick to the brace of the lapping tool.  
Stick with the Wheeler product from Brownells.

It wasn’t available when I tried to buy it.  It’s now back in stock but has to be bought with other grits I wouldn’t have needed.

I blacken the lapped surface with Aluminum Black, and oil it. That, plus the parkerized, nitrided, or stainless mating surface finish on the barrel extension mitigates the galvanic issue, or at least really slows it down.

Thanks for all the advice and replies. I was unable to find the garnet or the aluminum oxide compound actually as they're both out of stock at Brownell's now. I found this however:

https://www.amazon.com/dp/B07KWBZPG1?tag=arfcom00-20

Don't know what the intended purpose is, bit it should work on an AR face right?

What's funny is, this shows up at the bottom of that page: https://www.amazon.com/dp/B0063HCSOK?tag=arfcom00-20

This is a really simple operation.  You're just trying to level the face, not seat a valve or something where it might matter how fast the compound breaks down, or how mirror smooth the finish is.  Pretty much any aluminum oxide compound in the grit called for by brownells should work fine.

I just lapped my receiver ring with Clover 240 silicon carbide and it worked great.  I started with 320 and it was taking forever (with the Brownell's lapper) so I switched to a faster grit and got a perfect result.  Seems like 600 will probably take you forever, but it is a lifetime supply from Amazon.

The rear sight was bottomed out to the left so I hope this provides some relief when I go to the range this week.