Lapping Question??

[Keef]

I like lapping uppers. In my experience it makes swapping optics much easier. I bought a lapping tool because of a radical upper, I had to adjust the rear sight all the way to one side to get sighted in properly. Tore it down lapped it, reassembled, and when I sighted it back in the rear sight was sitting right in the middle like normal. This is an extremely case(maybe).

[Bdlot]

Well, anyway I look at it, I look forward to completing my first build in the near future and appreciate all your inputs. I will have to wait and see the fit anyway.
Thanks again for the support.

[ricsmall]

Bdlot

If you do end up needing to bed the barrel to upper, apply a couple coats of kiwi neutral shoe polish to the barrel extension. Makes disassembly much easier in the future. I’ve noticed the difference on a couple builds over the years. As already mentioned, if you’re fortunate enough to get a tight fitting combo, no need to bed it.

ETA: I lap all the uppers I assemble. Too easy to do not to do it. Eases stress on bolt lugs and as keef said, it keeps your sights/optics centered or nearly so.

[Mad Charlie]

"ETA: I lap all the uppers I assemble. Too easy to do not to do it. Eases stress on bolt lugs and as keef said, it keeps your sights/optics centered or nearly so."

He's right! I have slid a scope/mount combo off of one rifle and on to another and was only a little less than an inch off a 100. I just put it down to luck and had an extra brew that night, but both uppers were lapped.

[just_john]

When I built my first Grendel, there was no discussion about lapping. And the mounted barrel had a visible cant to the left ( once I started wondering why there was pronounced horizontal shifting as distances changed ). At this point, I have assembled or help other folks assemble over 50 ARs. All forged ( I don't do billet and don't encourage others to do so either ). And I true the extensions on all of them whether they need it or not. And almost all of them have needed it. The tool is cheap enough. The amount of time required is minimal and the potential results are certainly worth it.
Patrick Sweeney had an excellent article on accurizing ARs in a G&A issue about a year or so ago. And he stated that the greatest improvement that he made was trueing the receiver extension. I have used a lot of different brands of uppers. So far only the BCM uppers have been true out of the box.
It's cheap insurance. In the time reading / writing this discussion you could have trued several receivers. And it could bring a 1MOA rifle into a .5MOA shooter. Do it!

[Bdlot]

Thanks
I am convinced on lapping. It definitely is a sound and practical process. Bedding as well, if needed. I like the release agent use (ricsmall).

[lazyengineer]

When I built my first Grendel, there was no discussion about lapping. And the mounted barrel had a visible cant to the left ( once I started wondering why there was pronounced horizontal shifting as distances changed ). At this point, I have assembled or help other folks assemble over 50 ARs. All forged ( I don't do billet and don't encourage others to do so either ). And I true the extensions on all of them whether they need it or not. And almost all of them have needed it. The tool is cheap enough. The amount of time required is minimal and the potential results are certainly worth it.
Patrick Sweeney had an excellent article on accurizing ARs in a G&A issue about a year or so ago. And he stated that the greatest improvement that he made was trueing the receiver extension. I have used a lot of different brands of uppers. So far only the BCM uppers have been true out of the box.
It's cheap insurance. In the time reading / writing this discussion you could have trued several receivers. And it could bring a 1MOA rifle into a .5MOA shooter. Do it!

?
Not trying to be a wise-ass; just trying to figure out your post and process. I'm confused, because this is an AR15 "receiver extension".

I'm guessing that's a typo?

[Wheelhorse]

I bought a home built AR from a guy at a gun show once that had to have the rear sight all the way to the left to zero. When I lapped the upper, I had to take a lot of material off one side and when it cleaned up evenly all the way around, I put it back together and shot it. Then when zeroed, the rear sight was in the center.
I had a Colt HBar once that had to have the sight all the way over and I returned it to Colt. They did nothing and sent it back claiming it was "in spec".
I lap all of mine. Most are very close but they all get it anyway. I'm a believer.

[LRRPF52]

The pin might make it a little hard to lap the extension...
Most uppers are cast. There are some "billet" uppers. I have no experience with billet, all four of my uppers have needed lapping.

I'm not aware of any companies today that are casting uppers.

Uppers are either forged and machined, or machined from billet.

[LRRPF52]

From a machining perspective, especially in this day and age where everything is done on CNC, you get true receiver face surfaces......before the anodizing is done.

This is where the problem comes into play. During the anodizing process, it is pretty common for build up of material to occur unevenly.

This is why we lap the upper face, and why we don't need to mess with quality barrel extensions normally.

Barrel extensions aren't anodized since they are steel alloy. Not all barrel extensions are created equally, and most on the market do not know about or adhere to the TDP in materials, processes, and dimensions.

I really like BAT Machine extensions, which are held to very tight tolerances in concentricity, uniformity, and dimensional call-outs.

I do know a gunsmith who trued and squared his barrel extensions though before attaching them to the barrels.

His 6.5 Grendels shot .5 MOA out of the gate.

We've seen most samples of the cheaper barrel options out there fail to meet basic dimensional call-outs from the TDP regarding OD, chamfer for the lugs inside the lock-up area, threads undersized, feed ramps way off of the land marks, and basically every dimension very loose from where they are supposed to be.

Just because something looks like a barrel extension doesn't mean it even halfway meets the criteria that has been developed over many decades of hard lessons learned.

[Wheelhorse]

From a machining perspective, especially in this day and age where everything is done on CNC, you get true receiver face surfaces......before the anodizing is done.

This is where the problem comes into play. During the anodizing process, it is pretty common for build up of material to occur unevenly.

This is why we lap the upper face, and why we don't need to mess with quality barrel extensions normally.

Barrel extensions aren't anodized since they are steel alloy. Not all barrel extensions are created equally, and most on the market do not know about or adhere to the TDP in materials, processes, and dimensions.

I really like BAT Machine extensions, which are held to very tight tolerances in concentricity, uniformity, and dimensional call-outs.

I do know a gunsmith who trued and squared his barrel extensions though before attaching them to the barrels.

His 6.5 Grendels shot .5 MOA out of the gate.

We've seen most samples of the cheaper barrel options out there fail to meet basic dimensional call-outs from the TDP regarding OD, chamfer for the lugs inside the lock-up area, threads undersized, feed ramps way off of the land marks, and basically every dimension very loose from where they are supposed to be.

Just because something looks like a barrel extension doesn't mean it even halfway meets the criteria that has been developed over many decades of hard lessons learned.

I glad I read this post. I know I've seen them off but didn't know why. This makes perfect sense.

[Mad Charlie]

I'm not aware of any companies today that are casting uppers.

Uppers are either forged and machined, or machined from billet.

I did mean to say forged, thanks, will correct.

[ARErector]

OK, I'm convinced. I ordered a Wheeler lapping kit.

I have seen some of my builds with the sights adjusted off center and this makes sense now. Most of my builds were budget basement parts and they worked fine, but accuracy wasn't usually that great. When I tried to build a precision AR and spent considerably more money on parts and it came out shooting about the same as all the others, I was understandably disappointed. This and some other tricks I've learned here will hopefully help in the future.

[Ganderzone]

I have X-caliber's BART 2.0 tool, little pricey but watch for a coupon code on their FB or web site, will save you 15-20$. Awesome tool for lapping and reverse the tool and lock it to a vise to work on a upper, no stress on the upper itself. Little pricey but if you have multiple ARs and work on them it's worth it over the wheeler.

[ARErector]

I have X-caliber's BART 2.0 tool, little pricey but watch for a coupon code on their FB or web site, will save you 15-20$. Awesome tool for lapping and reverse the tool and lock it to a vise to work on a upper, no stress on the upper itself. Little pricey but if you have multiple ARs and work on them it's worth it over the wheeler.

Good find. I've wanted to get a barrel extension tool as well. I already ordered the Wheeler, so I'll just use that and get a separate torque tool later.