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Ar-10 Barrel Extension Threads?

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seanb

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#1
I have been threading AR-15 barrels for barrel extensions for a number of years.

I am going to try an AR-10 barrel now.

I have not been able to find any info on thread specs for the barrel extension from any manufacturer and or gap between bolt and barrel face etc.

I plan on using a dpms pattern extension in my armalite ar-10.

I learned that as long as you use a dpms pattern bolt you can use a dpms pattern barrel extension in an armalite.

I would use armalite but have not been able to find an extension for sale in the last 6 months
 

wrat

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#2
I have been threading AR-15 barrels for barrel extensions for a number of years.
Needing to start a project like that, myself. Any pointers? I decided maybe my Atlas lathe wasn't quite up to threading and profiling a blank.

I learned that as long as you use a dpms pattern bolt you can use a dpms pattern barrel extension in an armalite.
As i'm sure you know, but for the benefit of anyone just tuning in, the AR-15 is a standard. It's tightly controlled in almost every respect from dimensions back to where the bauxite came out of the ground. However, the AR-10 has no such standard. Most of the stuff is interchangeable -- trigger group, buffer tube, and stock, for instance -- but some stuff is definitely not.

DPMS is far and away the most popular "standard" for AR10, but that's not because of special qualities or accuracy in an of themselves. The one thing they did was to make the Magpul Pmag their choice in magazines where some of the other selections (like Knight) were built around things like FAL mags or M14 mags. Those configs have more or less evaporated these days.


Wrat
 

planeflyer21

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#3
Wrat is correct.

Terminlology is HUGELY important when working with 308AR rifles. You can easily end up with the wrong specs or incompatible parts, as there is NO milspec.

Your three basic 308AR platforms are Armalite AR10 pattern, DPMS rifles (the original and now the G2 patterns), and the Rock River Arms LAR-8. DPMS has far and away the largest support in aftermarket. Armalite has even come out with a "P-Mag compatible" lower, which is a way of saying "DPMS-style magazine" without saying DPMS.

Seanb, you are correct that the barrel extension and bolt must be of the same type (either Armalite OR DPMS). Mixing and matching will lead to major headaches.
 

MBuechle

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#4
BAT machine makes an AR-10 barrel extension, Brownell's sells them, very well machined. I used one on a .22-250 barrel for my Armalite. If you're finishing the barrel, you can mix bolts and extensions as you're in control of the bolt clearance and head space. Bolts & carriers should not be mixed though. This drawing was provided to me by a guy who claims to have taught John Hollinger the trade, I have no reason to doubt him. I've used and these dimensions work.
ar10_chamber.jpg
 

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Brac

Swarf
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#7
I realize this is kinda an old thread, but it really works with what I'm working on. I acquired a barreled Armalite receiver set. My plan is to turn a new green Mountain barrel using the armalite barrel extension. I have a new DPMS
BCG that I wish to use, and just hoping someone can verify for me, that if the barrel is made to headspace properly on the bolt the difference between barrel extensions is not an issue. Thanks
 

Msalm

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#8
Correct on the barrel extension. I have a bunch of the Armalite extensions, but prefer the ones made by BAT machine. If you thread a well fitted tenon, oftentimes the Armalite will not bear evenly around the shoulder...that NEVER happens on a BAT extension.
 

ghostdncr

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#9
I spent 2016 building an M110 and was amazed at the lack of standards on these big AR's.. This was nothing like putting together an AR15, to say the least. The original M110 was built by Knight's Armament using the Armalite pattern components, but I went with the DPMS standard for mine in order to save a huge amount of money. I ordered a Ballistic Advantage 7.62 NATO barrel early on and fortunately, it came already fitted with a very nice barrel extension. My BATF Form 1 was approved several months ago, so the next phase of this project will be getting started on the very unique suppressor for this rifle!

11_4_2016.jpg
 

BlackHornet

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#10
Sorry for bringing up an "older" thread but I have a few questions. I am about to rebarrel my AR308 in DPMS bolt pattern and this is the only drawing I could find online that remotely gave any dimensions.

The shoulder that has the diameter of .920"....is that shoulder always needed and does it need to always be .22"?

The drawing also shows that the case needs to stick out of the chamber .145"....does that always need to be that measurement?
On a bolt rifle, one would measure the distance from the edge of the receiver/recoil lug down to the bolt face and then add a clearance amount. I just dont know what the clearance should be on a gas gun from the front of the bolt lugs to the barrel.

I have rechambered many bolt rifles and so I know what kind of clearances are needed for those but chambering an AR308 is new to me.
 

Msalm

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#11
Measure your extension with the bolt locked in just like you would a bolt gun. Headspace and tenon length can both be taken off those measurements. Only have to add the chamfer/cone for feeding.
 

dave2176

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#15
Sorry for bringing up an "older" thread but I have a few questions. I am about to rebarrel my AR308 in DPMS bolt pattern and this is the only drawing I could find online that remotely gave any dimensions.

The shoulder that has the diameter of .920"....is that shoulder always needed and does it need to always be .22"?

The drawing also shows that the case needs to stick out of the chamber .145"....does that always need to be that measurement?
On a bolt rifle, one would measure the distance from the edge of the receiver/recoil lug down to the bolt face and then add a clearance amount. I just dont know what the clearance should be on a gas gun from the front of the bolt lugs to the barrel.

I have rechambered many bolt rifles and so I know what kind of clearances are needed for those but chambering an AR308 is new to me.
I don't think you need the shoulder. I did mine without it because I didn't know about it. The problem is you run into thread issues because the extension thread is not quite complete. You need to file a bit of the thread at a minimum because it gets to tight. As for the .145 sticking out of the chamber, I simply measured the pocket in the bolt where the cartridge sits. I allowed a couple thousandths for the bolt to rotate without binding. Turned out great (I think). You can see it here: http://www.hobby-machinist.com/threads/my-ar10-build.45654/

Is that a drawing of a dpms extension or a armalite extension?
It is DPMS. Check the chart in the thread just above that list differences.
Dave
 
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