Gunsmithing AR-15 Barrel Extension Question

[GBMaryland]

Ok,

So, in short, I've got a barrel on my Armalite upper (with M4 feed ramps in the upper) that has standard AR-15 (non-M4) ramps.

I'm planning on replacing my forearm with a Larue 13.5.

While I've got everything apart, I'm thinking I just want to put an M4 barrel extension on.

So I called Stag Arms to see if they sell an M4 barrel extension for their 20 inch barrel (the one I've got on the rifle)... and they tell me that it must be installed by a gun smith.

I was unable to determine if this was because machining WILL be involved, or if they were more covering their butts.

I'm assuming that their own barrels will have extensions that are very similar between batches, and the removing the current one and replacing it should be a piece of cake.

...and that so long as I use a set of Go / No-Go gauges to verify things are proper, there shouldn't be any issues.

Given that the barrel is chrome lined, I don't see how there is anything that a gun smith is going to be able to do.

Am I full of crap?

GB

 

[David300]

Re: AR-15 Barrel Extension Question

Yes
The barrel would have to be indexed so the gas tube will line up along with the head space being checked and or rechambered..

 

[gunnergirl81]

Re: AR-15 Barrel Extension Question

Headspace is set off the ext. A new ext will alter that headspace, time differently, and as such cause the gas port to index differently. Now normally, you could use a lathe to set the shoulder back a few thou at a time until the new extension times right and then reset your headspace with a reamer. But why would you do that with a chrome lined barrel (bye bye reamer)? I would either get an upper without feedramps, or a new barrel with an M4 extension. It is not an easy thing to do. Hell, its a pain in the ass to get an extension off to begin with. Good luck in whatever you decide.

 

[JelloStorm]

Re: AR-15 Barrel Extension Question

I have always wondered what the difference between M4 vs. non-M4 feed ramps is.

Can anyone explain that? I can't imagine the ramps on different on the carbine version of the standard A3 model AR15 would have different ramps.

 

[GBMaryland]

Re: AR-15 Barrel Extension Question

Basically, an AR upper with M4 feedramps is basically an upper made for use as a select fire weapon. (eg an m16)

So I have an older Armalite upper that came with an HBAR 20 inch barrel, but that catch was that when I replaced the barrel I didn't realize that it had M4 ramps that start in the upper receiver, and finish in the barrel.

Normal, "civilian" style sporter ARs have no need of deep M4 ramps, as they're never going to be used as fully automatic weapons... So the feed ramps are only are cut into the barrel extension.

My understanding is that in very rare cases you can end up rounds jamming in the sharp lip formed in the gap between receivers M4 ramps and the barrels "rifle" style ramps.

I'm having a very difficult time buying, for a moment, that any manufacture is not make barrels and barrel extensions to precise specifications so that the extensions are interchangeable on their assembly line and with several thousands of an inch.

It just doesn't make any sense. Why would you spend money having expensive trained machinist fitting each barrel / barrel assembly when you can just make them to exacting specifications to begin with?

 

[GBMaryland]

Re: AR-15 Barrel Extension Question

FYI: The second picture down is an example of what my rifle barrel was like originally....

http://www.weaponevolution.com/forum/showthread.php?322-Armalite-M15A4-ISP-Carbine-Review

An example of the various manners in which uppers / barrel can be configured (which points out my current config as "bad") is here:

 

[gunnergirl81]

Re: AR-15 Barrel Extension Question

The M4 feedramp was part of the US Army's revamp of the M16 platform. Funny thing is the new and improved M16 born as the M4 was rejected by the USMC as not being as reliable as a rifle length op sys, and not meeting their standards for reliability. They originally went with the M16A4 instead. The new feed ramps are just slightly wider and deeper than the previous, and slightly flatter in angle. They supposedly promote straighter feeding, but I'm not impressed as they scrape brass more.

The barrels are assembled unported. The headspace and gas port are established after the extension and the index pin is installed. The headspace and gas port can then be done in mass production by automated machinery at that point. The barrel/extenson once mated were never meant to be parted, just a pain in the ass. Don't get me wrong, it can be done. But with AR15/M16 parts being as relatively cheap as they are the question is why go through all this?

 

[GBMaryland]

Re: AR-15 Barrel Extension Question

Well, when you put it that way it makes more sence...

If they drill the gas port after the fact, then it makes perfect sense.

I can go buy another barrel... that's not a big deal. Though, honestly, I have yet to have anyone totally feak out and tell me I'm going to have all sorts of problems. I initally got on this kick simply because the edges are fairly sharp where the barrel extension meets the cut slopes in the upper.

 

[gunnergirl81]

Re: AR-15 Barrel Extension Question

Some folks get lucky and have the bad setup but no problems. We had a guy who was building a varmint rig and wanted to use soft point ammo. They hit the lip on the extension created by the M4 upper receiver and notched the tip of the projo and pushed it back in the case. We offered to get him a non-M4 receiver and swap it for his, but he chose to dremel the extension and it came out really craptacular.

 

[B y r o n]

Re: AR-15 Barrel Extension Question

If you have a setup like the "Bad" example there is a simple fix that you can do yourself. Using a Dremel tool or even a round file extend the bottom slots to match the contour of the receiver. Work slow and check frequently so you don't over do it. When done it should look like the upper right "Good". Be patient and you shouldn't have a problem. If you are uncomfortable doing it yourself you should be able to find a local smith that can do it for you.

 

[McLarenross]

Re: AR-15 Barrel Extension Question

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: B y r o n</div><div class="ubbcode-body">If you have a setup like the "Bad" example there is a simple fix that you can do yourself. Using a Dremel tool or even a round file extend the bottom slots to match the contour of the receiver. Work slow and check frequently so you don't over do it. When done it should look like the upper right "Good". Be patient and you shouldn't have a problem. If you are uncomfortable doing it yourself you should be able to find a local smith that can do it for you. </div></div>
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