Lapping AR15 Uppers

DaemonAssassin · Dec 13, 2018

omegared24 said:
I'd like to thank DaemonAssassin for his generosity. He set aside time and resources to teach me how to lap an upper. He really knows his stuff.

I brought 2 BCM blems ($59 each...they come with forward assist and ejection port covers). I also brought 2 garbage blem uppers that I purchased a while back. They were super cheap at $29. AO Precision if I recall correctly. Not to be confused with Aero Precision.

Interesting lesson about quality. You could tell very quickly how much better the BCM product is compared to the cheaper option. It took no time at all to remove the anodizing from the AO upper. The BCM took a good bit of work to properly remove the anodizing.

We had to be really careful inserting the lapping tool in the BCM upper as it was a tight fit. The AO upper wasn't as finicky.

Here are pics of each brand after lapping. You'll notice the BCM upper still has a little anodizing on it.

You're welcome! Happy to see that you like the work that was done and you picked up a few new pieces of information to use. If you need to use my bench or tools in the future, don't hesitate to reach out.

omegared24 · Dec 13, 2018

DaemonAssassin said:
You're welcome! Happy to see that you like the work that was done and you picked up a few new pieces of information to use. If you need to use my bench or tools in the future, don't hesitate to reach out.

It is appreciated!

omegared24 · Apr 8, 2019

Ran into something interesting with these cheap uppers that I lapped. Was looking at the milled out portion that made it a "blem" and it looks like one of them wasn't milled down far enough and found "A 3S679" in that spot. Tried taking a picture but couldn't get it legible. I just pulled a stock photo to show where the mill mark is.

The 3S679 is FN's cage code. Don't know what the "A" stands for but it does have the square intact to the right of it which I believe is the BA forge mark.

Any thoughts by the local experts?

jrumann59 · Apr 23, 2019

To the OP if using a stainless steel barrel, please make sure to use a good bit of grease between the lapped bare aluminum and the collar on the barrel, though not likely galvanic corrosion can happen between raw aluminum and stainless steel.

ironpony · Apr 23, 2019

jrumann59 said:
To the OP if using a stainless steel barrel, please make sure to use a good bit of grease between the lapped bare aluminum and the collar on the barrel, though not likely galvanic corrosion can happen between raw aluminum and stainless steel.

Will grease do the trick vs. the anti-corrosion special compounds. I've used quite a bit of these working in the marine industry Tef-Gel, Raymarine has their own variety etc.

jrumann59 · Apr 23, 2019

ironpony said:
Will grease do the trick vs. the anti-corrosion special compounds. I've used quite a bit of these working in the marine industry Tef-Gel, Raymarine has their own variety etc.

Aero-shell grease should do the trick you just want a nice barrier between the dissimilar metals.

clandestine · Apr 23, 2019

Bedding with Loctite 620 prevents dissimilar metals from making contact.

Occam · Apr 23, 2019

Visit the below thread on Clandestine's SOTAR IP page. There's an embedded video from a build class. You'll see some of this lapping/bedding and dremel/ramp action right after 1:30 in that short video.

https://www.mdshooters.com/showthread.php?t=231714

jrumann59 · Apr 23, 2019

clandestine said:
Bedding with Loctite 620 prevents dissimilar metals from making contact.

I am talking about the where the barrel collar meets the lapped face.

omegared24 · Apr 23, 2019

clandestine said:
Bedding with Loctite 620 prevents dissimilar metals from making contact.

This is what I'll do but I'm more concerned about using these blems. Who knows what the QC was on that thing.

clandestine · Apr 23, 2019

jrumann59 said:
I am talking about the where the barrel collar meets the lapped face.

Yep. The 620 oozes and fills where the Barrel Extension shoulder crushes against the receiver.

3paul10 · Apr 23, 2019

clandestine said:
Yep. The 620 oozes and fills where the Barrel Extension shoulder crushes against the receiver.

...I love it when Chad gets talking dirty....

DaemonAssassin · Apr 30, 2019

3paul10 said:
...I love it when Chad gets talking dirty....

It makes me moister than an oyster.

trickg · Mar 18, 2022

I thought I'd bump this thread. I got interested in this from a Classifieds post for an upper that has been "hand lapped," so I did some reading.

You've got to love the subject of the AR15. Everyone has their own personal methods of doing things (or are doing it that way because company "X" supposedly does it that way) and everyone is absolutely married to their method.

It's hard to tell what's what. Take just the simple subject of installing the barrel into the receiver. I've read:

Barrel extension needs to be dry - you want to it lock down and not move
Thin coating of oil
Thin coating of whatever lubricant is nearby
Thin coating of Aeroshell 33
609 Loctite
620 Loctite

I've also read that there needs to be some kind of barrier due to possible galvanic reaction? That would completely negate the idea that the barrel extension needs to go in dry, wouldn't it?

Initially I installed it dry, but when I took it apart to put on the personalized dust cover (which wouldn't come off unless I took the barrel nut off) I coated it with Aeroshell when I put it back together.

And now I'm reading that I could (theoretically) improve the accuracy by lapping the face of the upper receiver and bedding the barrel with 609 Loctite. I don't want to mess with it though - it's already getting 1 MOA, give or take.

rbird7282 · Mar 18, 2022

trickg said:
I thought I'd bump this thread. I got interested in this from a Classifieds post for an upper that has been "hand lapped," so I did some reading.

You've got to love the subject of the AR15. Everyone has their own personal methods of doing things (or are doing it that way because company "X" supposedly does it that way) and everyone is absolutely married to their method.

It's hard to tell what's what. Take just the simple subject of installing the barrel into the receiver. I've read:

Barrel extension needs to be dry - you want to it lock down and not move
Thin coating of oil
Thin coating of whatever lubricant is nearby
Thin coating of Aeroshell 33
609 Loctite
620 Loctite

I've also read that there needs to be some kind of barrier due to possible galvanic reaction? That would completely negate the idea that the barrel extension needs to go in dry, wouldn't it?

Initially I installed it dry, but when I took it apart to put on the personalized dust cover (which wouldn't come off unless I took the barrel nut off) I coated it with Aeroshell when I put it back together.

And now I'm reading that I could (theoretically) improve the accuracy by lapping the face of the upper receiver and bedding the barrel with 609 Loctite. I don't want to mess with it though - it's already getting 1 MOA, give or take.

My .02
The upper should be lapped just until it’s trued.
The barrel extension should be bedded with Loctite 620.
The threads should be coated with Aeroshell 33
and torqued to proper spec.

Definitely have to use Aeroshell (or equivalent) to prevent galvanic corrosion.

Sent from my iPhone using Tapatalk

alucard0822 · Mar 18, 2022

trickg said:
I thought I'd bump this thread. I got interested in this from a Classifieds post for an upper that has been "hand lapped," so I did some reading.

You've got to love the subject of the AR15. Everyone has their own personal methods of doing things (or are doing it that way because company "X" supposedly does it that way) and everyone is absolutely married to their method.

It's hard to tell what's what. Take just the simple subject of installing the barrel into the receiver. I've read:

Barrel extension needs to be dry - you want to it lock down and not move
Thin coating of oil
Thin coating of whatever lubricant is nearby
Thin coating of Aeroshell 33
609 Loctite
620 Loctite

I've also read that there needs to be some kind of barrier due to possible galvanic reaction? That would completely negate the idea that the barrel extension needs to go in dry, wouldn't it?

Initially I installed it dry, but when I took it apart to put on the personalized dust cover (which wouldn't come off unless I took the barrel nut off) I coated it with Aeroshell when I put it back together.

And now I'm reading that I could (theoretically) improve the accuracy by lapping the face of the upper receiver and bedding the barrel with 609 Loctite. I don't want to mess with it though - it's already getting 1 MOA, give or take.

Much of it comes down to the quality of the parts to begin with, bedding and lapping might help a loose fitting barrel in a upper with loose specs, but probably not much difference in one that is already concentric. I usually lap and bed my precision stuff, but just check most, run the lap lightly and if it looks like it is hitting the face flat, I'll leave the anodizing alone. In either case probably the most important is to use something good like Aeroshell 33 to lube the barrel nut threads and face, then torque and back off, then torque again. Without the grease on the nut, it can seize up, apply torque to the locator pin and damage the upper or break the pin.

3paul10 · Mar 18, 2022

rbird7282 said:
My .02
The upper should be lapped just until it’s trued.
The barrel extension should be bedded with Loctite 620.
The threads should be coated with Aeroshell 33
and torqued to proper spec.

Definitely have to use Aeroshell (or equivalent) to prevent galvanic corrosion.

Sent from my iPhone using Tapatalk

That's it in a nut shell. Nough said.

outrider58 · Mar 18, 2022

rbird7282 said:
My .02
The upper should be lapped just until it’s trued.
The barrel extension should be bedded with Loctite 620.
The threads should be coated with Aeroshell 33
and torqued to proper spec.

Definitely have to use Aeroshell (or equivalent) to prevent galvanic corrosion.

Sent from my iPhone using Tapatalk

^^^This is the way. :thumbsup:

alucard0822 said:
Much of it comes down to the quality of the parts to begin with, bedding and lapping might help a loose fitting barrel in a upper with loose specs, but probably not much difference in one that is already concentric. I usually lap and bed my precision stuff, but just check most, run the lap lightly and if it looks like it is hitting the face flat, I'll leave the anodizing alone. In either case probably the most important is to use something good like Aeroshell 33 to lube the barrel nut threads and face, then torque and back off, then torque again. Without the grease on the nut, it can seize up, apply torque to the locator pin and damage the upper or break the pin.

Out of the dozen or so uppers I have lapped, I have never run into one that was factory true.

3paul10 said:
That's it in a nut shell. Nough said.

Can I get an amen?

trickg · Mar 18, 2022

Ok, this is all good stuff - at least there seems to be some consensus on this forum. :lol2:

Regarding the lapping tool, is the Wheeler tool decent enough, or are there others that are better? I'm not opposed to spending some money on tools if I feel it's warranted, although I don't know how many more ARs I'm going to build.

How necessary is it to bed the barrel? Very necessary? Necessary only for improved accuracy? Just curious.

The barrel on this first build was pretty snug in the upper, which was a PSA blem upper that I got on the cheap years ago and finally decided to put to use. All I know is that the rifle is a pretty solid shooter - my son and I are doing 20 shot groups at 50 yards that aside from the odd flyer which is probably our fault, are slightly bigger than a quarter. Is that super accurate? Nope, but that could also be attributed to the ammo - 55 gr reloads out of a 1:7 twist barrel. (1:9 would be better for 55 gr)

I'll employ some of this on the next build - I've got all the parts on hand or in the mail, so I'll probaby pick up a lapping tool. The upper on the way is an Aero upper, so hopefully it's pretty solid and won't need a lot of work, and hopefully the barrel fits pretty snugly.

bigjohn · Mar 18, 2022

Since I got my lapping tool, I lap all my uppers. I don’t bed them tho. They all are 1 moa or less

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