Lapping Question??

[Bdlot]

This could be a not so smart question, but I am seeing more and more about lapping your upper receiver.
The upper is a machined part, and your barrel extension is also machined.
Some run out and buy lapping compound and tool and lap the upper, but am I missing the part about the barrel extension that we are assuming that it is true?
Is it necessary to lap an Aero Precision upper?
I am still looking for a 16" barrel or I would ask if the barrel is of concern.
Thoughts?

[Mad Charlie]

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

[A5BLASTER]

I lapp any and all uppers. Not so much cause I think it makes it more accurate but more so to keep the parts square for the bolt lock up and even stress on the bolt lugs.

[Bdlot]

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.

Thanks for the reply Mad Charlie.
I understand the index pin could hinder this process. I am just trying to justify lapping using other's experience(s).

[Bdlot]

I lapp any and all uppers. Not so much cause I think it makes it more accurate but more so to keep the parts square for the bolt lock up and even stress on the bolt lugs.

I guess I should have thought about this earlier as I now have 2 enhanced uppers (M4E1 and a M5E1), but I do see they sell lapping tools for the enhanced uppers also.
Thanks for the help folks.

[grayfox]

I had a barrel/nut/GB I could not for the life of me dis-assemble, didn't need it for anything but needed the nut... so cut it in half.
Then I took the extension-half, sanded/smoothed down the alignment pin and Viola! I've got a hand-based lapping extension that I can use. As for the fwd end of the upper, when you lap with compound you take off realistically less than 0.001" to square it up. So basically no impact on the upper's overall performance except for squaring off the mate-up to the barrel

[Bdlot]

I had a barrel/nut/GB I could not for the life of me dis-assemble, didn't need it for anything but needed the nut... so cut it in half.
Then I took the extension-half, sanded/smoothed down the alignment pin and Viola! I've got a hand-based lapping extension that I can use. As for the fwd end of the upper, when you lap with compound you take off realistically less than 0.001" to square it up. So basically no impact on the upper's overall performance except for squaring off the mate-up to the barrel

Thanks,
Santa is going to have to get on this tool shortage situation I am in. I appreciate all the help.

[VASCAR2]

If you have a level smooth surface like granite table top you could stand the receiver on the front where the barrel is inserted. You could then use a light to to see if the receiver has any gap or is not level and square with the granite surface.

I have a gunsmith friend who has a lapping tool and a mill. We always lap my upper receivers but like grayfox mentioned we have only removed a small amount of material. I like having the barrel square with the receiver so there is no uneven pressure on the bolt. I always bed my barrel and upper receiver with blue Loctite and use blue loctite on my gas block and screws. I’ve done several uppers this way and they all shoot good and I haven’t had any issues with my gas blocks moving or coming loose.

[lazyengineer]

I ordered a lapping tool to try this out. I'm not convinced one way or another on if this actually matters at all; but hey - my accuracy is a touch less than I'd like, and it's not all that expensive to try this.

Also, i consider the Grendel less forgiving for durable life, so if this helps assure a more square mating of the bolt to the barrel extension, that's probably a good thing. So, I spent the $30 and will invest the 2 hours or so of dismounting the barrel and everything, lapping, and then rebuilding the upper again, and we'll see.

[grayfox]

If you have a level smooth surface like granite table top you could stand the receiver on the front where the barrel is inserted. You could then use a light to to see if the receiver has any gap or is not level and square with the granite surface.

...

Economical genius!!! I like it...

[Bdlot]

Thanks for all the info folks.
Are all of you bedding also?

[grayfox]

I did it once.... then had to break the build apart. sheeeesh.
Didn't see any big improvement, but then again I'm not an expert marksman.
I think more important than bedding per se is use an upper that is tight -- like BCM or maybe Aero, so you have to heat it up to get the fit. Then bedding is not needed so much.
Cheaper -- Anderson et al -- uppers, bedding could be helpful. But you're only talking 30-40 bucks if you shop around. Why mess with slop anyway?
My 2 cents'.

[Klem]

Thanks for all the info folks.
Are all of you bedding also?

No, tried it once but it made no improvement to group sizes. My thoughts are that it is only likely to improve results if there's existing clearance between the two parts. Filling the void of a sloppy fit with proxy metal makes sense.

If you get quality parts with tight tolerances and good quality-control you won't need to fix anything later.

[imaguy3]

I personally don't think it's worth it... I have two factory ARs in 223 shooting factory ammo to 1moa or better.. My grendel shot factory ammo right at moa.. then I started loading for it.. I think it's more important to tune or find a load your gun likes..

[Mad Charlie]

"If you get quality parts with tight tolerances and good quality-control you won't need to fix anything later. "

I agree with this statement completely! However I haven't hit that lottery yet. In four upper builds only one had a decent tight fit, and the worst was an AA upper and AA bbl, .0045 slop. Most have run .002-.003

When you order parts from various suppliers you either build with what you get, or you spend money and time shipping parts back and forth. No way to try out parts for the fit up you want.

So I just bed and be done with it, same as I just lap and be done with it. Not hard to do and doesn't take long to do it, and then I'm sure it's done as best as I can do it.

If I ever got an upper that I had to heat and a bbl. that I had to chill to fit up, I might do somersaults or something, and since I can hardly walk that could be hilarious.

[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!