AR Upper reciever ID spec

[ucb]

I have a mega billet upper (m4 feeds)
DD CHF barrel (M4 feeds)

The barrel doesnt full insert into the upper (gets about 5-6mm away before seizing) . I measured the barrel OD, its right at 1.00" as it should be for an M4 profile

So checked the ID of the receiver, its 1" max and a few thousandths under in some areas.

So my question is, what is the standard tolerance/spec for the ID. Im guessing it should be 1.002 + 0.002/-0.00 or something like that.

[bender152]

I built an upper with a Noveske barrel and a Vltor receiver that had very close tolerances.

I lightly tapped in the barrel with a mallet. Only took a few taps for the barrel to go in.

[ucb]

Tried that, even fully cleaned with some lithium grease

Its ok, I called mega, they said they'd replace it. Although they did mention they machine them a little smaller than most.

[Argonaut]

I might try some dry ice on the barrel prior to fitting

[bender152]

That's some good customer service by Mega. And perhaps they've run into this before since they did mention that they machine them a bit tighter.

[shark92651]

I had a tight fit last night between a DD barrel and the mega monolithic but was able to tap it in using a little lithium grease and a rubber mallet while the upper was in the panther claw in the vise.

[ucb]

Yeah im not sure if I want to do that. Barrel removal in the future will be a pain. And in my case, light tapping didn't do the trick

[ar15barrels]

A tight fit between reciever and barrel extension is a GOOD thing.
Hit the reciever with a heat gun or torch to get it to about 300 degrees.
It will open right up.
Slip the barrel in and it will shrink back into place as it cools.

Barrel extensions should be 1.000" -0.0005" / +0.0000"
Recievers should be 1.000" -0.0000" / +0.0020" for the first inch of the reciever bore, starting from the front.
Then the reciever bore steps up to 1.0070" where the bolt carrier runs.

[CHS]

I've noticed that DD barrel extensions seem to be a bit on the large side, and are tight in most uppers.

Do what Randall suggested and/or hit it with a rubber mallet after applying anti-seize grease to the extension.

[Joe788]

You could also just use some lapping compound and knock a few tenths off of the anodize in about a minute or two.

[ucb]

Thanks for the info Randall. I've measured the barrel, its 1.00" +0/-0.003

Receiver is 1.00" MAX. Under by a few thousandths in a few areas depends on how I place the calipers.

So it looks like the combo of a very slightly over-sized barrel perhaps and a very slightly under-sized receiver. I'll try a few things tonight. If im not happy with the results I'll send it back to Mega. They also suggest the heat-treatment

[ar15barrels]

So it looks like the combo of a very slightly over-sized barrel perhaps and a very slightly under-sized receiver. I'll try a few things tonight. If im not happy with the results I'll send it back to Mega. They also suggest the heat-treatment

Just heat it up and slide it together, don't bother with anything else that would remove metal.
If you ever need to get it back apart, the same thing works because aluminum expands much more than steel at the same temperature.

[Calif Mini]

AR Upper receiver ID spec

Listen to Randall as he has done this a bunch of times.

I can, and have recommended him to other Cal-gunners needing help with critical items that if done wrong may ruin parts permanently.

Anything that removes metal on critical fitted parts, that is not controllable, has got to be used as a last resort.

I was thinking of hot (boiling) water. A torch, in the hands of someone inexperienced, can ruin the finish at the least, and even melt sharp edges if you fall asleep at the switch. The tip of the flame on a torch is 3 times hotter than needed to melt aluminum.

[ar15barrels]

I was thinking of hot (boiling) water.

Boiling water would probably work too, just make sure that the reciever comes up to temperature enough that all the water steams it's way off before you slide the barrel in.
No need to trap unnecessary moisture in between the barrel extension and the reciever.

[aplinker]

Hair dryer.

Use the diffuser attachment to prevent split ends.