AR barrel ?

David J. · Apr 11, 2013

I'll leave out the background to the question at this point it doesn't matter.
How much pressure does the barrel retaining nut have on it when a 5.56 is
fired? Or how hard is the barrel being "pushed" forward away from the upper?
I looked up max nato pressure as around 62,000 psi. the cartridge base area
is maybe .11in....so maybe 6900 lbs range? So how much of this becomes
forward pressure on the barrel?
Kinda lost on this concept, looking for insight or something to help
me understand the physics of this.
Links to a discussion of this would also be great.
Thanks
David

ditchdigger · Apr 11, 2013

The bolt locks to the barrel extension which is threaded on to the barrel. No pressure on the upper. That's why it's made of aluminum.

Cole2534 · Apr 11, 2013

So why do you ask? If you're wondering, aluminum barrel nuts survive just fine, ala Knights URX.

David J. · Apr 11, 2013

I ask because someone wants me to bore out and thread a existing aluminum barrel nut to
fit the oversize 1 5/16in thread pattern on a Bushmaster Carbon 15 upper. I don't have a problem
machining the barrel nut but have concerns about how it will survive. As mentioned above I
don't know AR rifles....they just asked me to open up the barrel nut. Not sure of the brand but
it is extended with external threads to retain some kind of quad rail.
So does the bolt actually lock into the barrel?
Time to get familar with the AR design.
Thanks
David

David J. · Apr 11, 2013

Looked at AR parts breakdown...tell me if I have this right. Looks like the barrel threads
into the barrel extension and the bolt rotates and locks to the extension when in battery.
I see a retaining ring that holds the extension to the upper??? If that's correct it looks like
the barrel nut mostly just keeps things from rattling loose??
Am I close to how this works?
Thanks again
Davi

Kyle Smith · Apr 11, 2013

---------------

gkoenig · Apr 11, 2013

The barrel nut is what holds the barrel assembly (barrel + barrel extension) to the upper receiver.

Chamber pressure has little/no effect on the barrel nut. Lots of companies make custom barrel nuts in order to attach various proprietary rail systems, and these are often made of 6061 without issue.

The Carbon 15 features a larger barrel nut ID, as the system requires more mating surface area and larger threads than an aluminum upper receiver. I take it your client is attempting to adapt a proprietary rail barrel nut to the Carbon 15?

David J. · Apr 11, 2013

Yup, he wants me to re-thread the 11/4x18 aluminum aftermarket barrel nut to install on the larger thread of the
carbon 15 upper. This barrel nut mates with some kind of quad rail / handguard.
I have doubts about the reduced wall thickness of the modified barrel nut.
I'm not dealing with the gun at all....just the nut, but lack understanding of how ARs are built.
Hence the doubts about the safety in doing this, gun owner tells me this is a somewhat common
modification.
Thanks
David

David J. · Apr 11, 2013

Kyle Smith said:
---------------

Huh???
David

Cole2534 · Apr 11, 2013

Without knowing more about the nut I can't say for sure how comfortable I'd be, but it doesn't appear that you're removing much material.

Now whether than 10 or 90% of the remaining nut is the question.

MIBill · Apr 11, 2013

David J. said:
I'll leave out the background to the question at this point it doesn't matter.
How much pressure does the barrel retaining nut have on it when a 5.56 is
fired?

It takes only the recoil force which is not much... 20 foot/pounds tops.

David J. said:
Or how hard is the barrel being "pushed" forward away from the upper?

It's not... The barrel, barrel extension and bolt deal with the chamber pressure. The barrel and barrel extension are being pushed away from the bolt and/or the bolt is being pushed away from them... Recoil is pushing all three parts into the the upper, not away from.

David J. said:
I looked up max nato pressure as around 62,000 psi. the cartridge base area
is maybe .11in....so maybe 6900 lbs range?

Your math is good, in round numbers... But irrelevant to your question.

David J. said:
So how much of this becomes forward pressure on the barrel?

Exactly zero psi as it relates to your question.

David J. said:
Kinda lost on this concept, looking for insight or something to help
me understand the physics of this.

Actually you got the physics nailed... But the part of the physics you are looking at ends with the barrel, barrel extension and the bolt. Think of it like this...

The barrel is attached to the barrel extension just like a barrel and receiver in a bolt action rifle. Think of those parts as if it were a bolt action and it gets a lot clearer. Your question is exactly like asking how much forward force is there on a Remington700 recoil lug... Answer: zero psi. The barrel nut only holds pieces together so that they go for a ride as a single unit force exerted by recoil.

David J. said:
Links to a discussion of this would also be great.
Thanks
David

David J. · Apr 11, 2013

Thanks for the info, most of my issue was due to my ignorance of ARs. I was thinking the bolt
locked into the reciever not the barrel extension....duh!
Between looking at pictures and parts views of the gun and the feedback you guys have
provided I think I've got the idea of what's going on.
I guess I'm one of the 4 or 5 people in the country who don't own some version
of the AR, never even fired one.
David

Cole2534 · Apr 12, 2013

They're pretty fun guns, everyone needs one!

Kyle Smith · Apr 12, 2013

David J. said:
Huh???
David

I typed a response, decided I didn't like it and deleted it. You have to have ten characters minimum.

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