AR-15 Upper receiver lapping?

[PNW_Steve]

Hello Everyone,

I am working on an AR build with an eye towards accuracy. I have started with the following:

Mega Billet Upper
Anderson Lower
S&W LPK
Odin BCG (6.5Grendel)
White Oak Barrel (.243LBC)
FAB Defence Stock

One of the reasons I picked the upper is that it requires "thermal-set" barrel fitting. You have to heat the upper to fit the barrel. I also want to true the upper receiver face. I have ordered the lapping tool from Brownells and while waiting I have done some reading....

It turns out that a number of folks are having trouble using the lapping tool because it won't fit in the receiver.

Would it be appropriate to heat my upper in order to fit the lapping tool?

Thanks.

S.

[popper]

Lapping just squares the face of the upper where it meets the extension ring. Heat fit upper tries to insure extension is square and tight. My factory LR308 has neither and is more accurate than me @ 200.

[cat-mechanic]

I didn't realize this was even a thing.

So I went to youtube and watched a few videos on it. Looks like all the ones I saw, the lapping tool just fit fine into the receiver. They would just lubricate the tool and it fit freely into the receiver.

If yours does not fit, I would start looking for problems. Measure the ID of the upper and the OD of the lapping tool. Then see how much space you will have.

I am afraid that if you heat your receiver, as it cooled, it could contract back to its original size and if you have the tool in place when this happens. It could seize the tool into the receiver.


AH, just hit me that you are using the "Thermal-set" upper. So yes, your upper receiver dimensions would be smaller than the tool.

If it were me, I would talk to a machinist friend and see if he could cut the tool smaller, to fit in your receiver, But then the tool would only work on that type of receiver.

Or put the tool in a drill press and work the shank evenly with some sandpaper. Might just take a little cleanup in order to get it into your receiver.

[LynC2]

If it won't fit without heating the upper I would not do that. It will most likely seize as soon as the upper cools. It would be best to have someone true the upper or turn down the lapping tool so that it would fit with clearance.

[M-Tecs]

I've never used it so I will not comment on if and how much it improves accuracy. I've chamber and built a lot of AR's with Krieger barrels and most are 1/4 to 3/8 MOA rifles. With a Krieger barrel I have yet to have one not hold 1/2 MOA. I do use a little Loctite on between the upper and the extension.

[Dimner]

I have a BCM upper which has an on purpose lower end of the spec barrel extension hole. For the purpose of a tight fit I bought the Brownell's receiver lapping tool and it worked just fine with mine. I lapped mine a month ago. It's super easy to do. I'd suggest using no power tools to lap the receiver face. I did mine by hand with valve lapping compound from the auto parts store. It came in a 2 pack with coarse and fine grits.

Did it make a differecent accuracy wise? I'm not sure, but two things to note:

1) the receiver face was not true at all. By how much? I don't know. But I tracked my progress using a black sharpie to see where spots were being ground and spots were not.

2) I tried 2 barrels before I lapped the receiver face. With awful results (8moa @50 yards with a scope on 12x magnification). Both barrels by a well regarded mfg. After lapping, I installed a 3rd barrel (received by mfg as a warranty replacement) I got 1moa at 50 yards ...then Installed an adjustable gas block, and now I am shooting .75 moa at 100 yards. With a ~$150 barrel.

So, did it help? Maybe? I can see how it could have, but also see how it may not have really done anything impactful.

I do know that this AR15 is for CMP shooting matches and I am going to wring every bit of accuracy I can out of this rifle while also not spending huge bucks on top of the line match parts. So it's up to the shooter to decide

[PNW_Steve]

Well it turned out to be much worry about nothing. The tool fit without a problem. It was a VERY tight fit but it did fit.

Observing the progress it looked like it was out of square a bit due to varying thicknesses of the finish.

The lapping went without any issues.

Now I have another challenge. When I tried to put my upper in the vice I found that my clam shell blocks don't fit this upper. They worked fine on my M&P-15. After finding that they didn't the Mega upper I checked my other uppers and found that they only fit the M&P.

I am considering putting a 1" diameter wooden dowel inside the receiver and facing the vice jaws with pine then clamp the upper in the vice. What do you all think?

[Dimner]

Personally, I bought a knock off "reaction" rod from ar15discounts. I think it was 29.99. I had a cappy clamsshell too that I had to use c-clamps on (while in my bench vise) in order to get enough holding power.

I don't think I'd want to try and clamp it kinda freestyle like you are talking about. Not if the operation you are going to perform means you are going to be torquing on it.

[JimB..]

I am considering putting a 1" diameter wooden dowel inside the receiver and facing the vice jaws with pine then clamp the upper in the vice. What do you all think?
I would not do that. Someone with more experience than I will be along shortly with a helpful suggestion.

[nicholst55]

Buy the Reaction Rod, or else pay someone that owns one to install your barrel. You'll be glad that you did.

[PNW_Steve]

Buy the Reaction Rod, or else pay someone that owns one to install your barrel. You'll be glad that you did.

I looked at the reaction rod but also read that securing the barrel instead of the upper receiver, when installing a barrel, puts too much pressure on the index pin and slot. I can see that. The index pin would be the only thing keeping the upper receiver from spinning when I try to torque the barrel nut.

I have never used a reaction rod myself so all I have to go on is what I hear/read from others.

Is securing the barrel instead of the upper receiver OK? What about the index pin and slot?

Thanks.

S.

[FLINTNFIRE]

If you are using that much force you are using to much , I have a set of armorers blocks for clamping the barrel in a vise , your upper was made to use the dpms panther claw vice block or one of the others just like it , You can do the barrel with a home made clamp if you want clamping the barrel , torque is only supposed to be 35 pounds or so .

[Dimner]

My reaction rod works exactly as you mention. I have used it for 3 operations.

1) Remove barrel nut
2) Install new barrel - Torque same barrel nut
3) Install muzzle device (flashhider/comp)

My barrel nut was torqued on with the old barrel within the military specifications of 35 foot pounds to 80 foot pounds. Since I installed the old barrel, I know it was torqued at 40-50 ft lbs. I installed my new barrel and reused the same nut and torqued around 55-60 ft lbs in order to get the holes lined up for the gas tube.

My muzzle device uses a crush washer to get it timed in the correct position, so after screwing it in freely, the crush washer starts doing its thing and it's a constant torque until I get to the orientation of the device that I want. I'm told the crush washer resistance is around 30ftlbs

After these operations, I looked at the barrel extension index pin slot on my upper receiver. I had the same hesitations as you. I did not notice any issues with the both the slot and the pin on my barrel extension.

The reason I decided to give it a try is: Per the advice of someone I respect on another forum, and verifying it with further google searches, the barrel extension is torqued on the chamber side of your barrel at 150ftlbs. Given that I never came close to that spec it would not cause harm to the index pin on the barrel extension. So if you have a decent quality barrel, and you have one of the best in the business(WOA), it was put together with mil spec tolerances/torques and it wont be moving anywhere under 100ftlbs.

This last part I do not have direct personal experience with, but I am told that getting to 100ftlbs of torque takes some very strenuous effort and will rely on a long cheater bar (leverage). So I don't think using normal tools any of us can come close to 100 ft lbs by accident.

[daengmei]

I am told that getting to 100ftlbs of torque takes some very strenuous effort and will rely on a long cheater bar

Take a look at this site. https://www.itorque.com/mechanical-dial-torque-wrenches

The specs listed along with the tool lengths give an idea of the force needed.

Now if your going 1000lbs, that takes a 77" long tool using this vendor's product.

[PNW_Steve]

While searching for answers I came across this item:

https://www.opticsplanet.com/vism-upper-receiver-block-for-ar-15.html

It looks like it should fit most uppers and will support the upper.

[Dimner]

Your mileage may vary, but I also saw those/that type of upper holder/vice. Decided to stay away from them for two reason that I read online (meaning, not first hand experience). 1) People, however few, reported that it caused some damage to the takedown holes/tabs on their uppers. 2) They do not fit every upper. I stayed away from them mostly because of the 2nd reason. I wanted to order something that I knew would work with my upper no matter what.

So if you get one and it fits, just make sure when you are doing a torque operation it's not putting a bunch of stress on the pins.

[1006]

I have only seen a friend use the lapping tool. He liked the physical results that he observed on a lower priced factory gun’s upper. But, the accuracy of the gun did not improve.