LR 308 Buffer TUBE question

[kenk]

30 minutes ago, Armed Eye Doc said:

The difference between a milspec and commercial buffer tube (receiver extension is the proper term) is the external diameter and only affects the stock that you can put on it.  The internal diameter should be the same with each.  See the diagrams in @jtallen83 post above.

The length of receiver extension can vary in a 308AR.  A proper AR15 receiver extension should be 7 inches deep internally.  DPMS 308ARs utilized the AR15 receiver extension of 7 inches.  Armalite AR10 receiver extensions are longer at 7 5/8 internal depth.  Either receiver extension can be used regardless of whether you have a DPMS or Armalite rifle.  You just need to be sure to use the proper buffer and buffer spring.

Thanks again guys, just one more thing. using the schematic provided by jtallen83, and other reiterations of the same info, I see that the threaded (receiver) side of the receiver extension is different between the commercial and MILSPEC versions. 1.170" com vs 1.185 M.S to be exact, now I know that the "teeth" or threads on the M.S versions are supposedly "sharper" and those on the commercial are "flatter"  is this what allows the two to fit up as you say?

My receiver extension is 7 5/8" long, and 1.70" in diameter (basically at both ends), when purchasing a new one online I am running into only ones labeled as MILSPEC, so to clarify, to your knowledge  as long as the buffer kit is for .308 cal, I can put it on this rifle?(I know no one is liable, just clarifying) Or am I totally missing it again? 

So I have the 7 5/8"  receiver extension, buffer, and spring that goes with it, If I replace the receiver extension with a 7" MILSPEC one how do I get the right size spring, the one I have would be too long right? and all of the ones I am seeing just say rifle or carbine, but..... as I am seeing I have a carbine receiver extension that is different dimensions than some......

thanks again guys, swear I am not an Idiot, just trying to eliminate room for error.......

[jtallen83]

1 minute ago, kenk said:

eliminate room for error.......

When it comes to the large frame AR's the best way to eliminate the chance of error is the Armalite kit;

https://www.armalite.com/SACItem.aspx?Item=AR10REKIT01&ReturnURL=/Armalite/Product-Category/AR10-Parts-Accessories/Lower-Receiver-Parts&Category=ac614400-ff09-4cdf-9d35-419a654e7201

[shooterrex]

The 7 5/8" extension you have will work with the Armalite carbine buffer and spring.

[98Z5V]

9 hours ago, kenk said:

Thanks again guys, just one more thing. using the schematic provided by jtallen83, and other reiterations of the same info, I see that the threaded (receiver) side of the receiver extension is different between the commercial and MILSPEC versions. 1.170" com vs 1.185 M.S to be exact, now I know that the "teeth" or threads on the M.S versions are supposedly "sharper" and those on the commercial are "flatter"  is this what allows the two to fit up as you say?

 

That's not something to worry about, and the difference in that drawing is there because the milspec uses rolled threads, and the commercial uses cut threads.

The only way to tell the difference isn't by looking at them - it's by measuring the O.D.  Mil=spec is 1.14" OD, and commercial is 1.17" OD. 

That will tell you right there what you have, right away.  Digital vernier caliper is the fastest way to find out.  Don't measure threads, and waste your time - measure the smooth part.  You'll know right away.

This should break it down pretty easy for you:

Edited July 19, 2020 by 98Z5V

[edgecrusher]

I really think he’s confusing the difference between the two and what it pertains to. It’s for your choice of stock and fitment there of. They make stocks for both sizes and you’ll want to choose the correct size for your extension. 
 

it does not have anything to do with the buffer or spring. That has to correlate with the internal depth of the receiver extension (aka buffer tube). Buffer and spring size matter depending on the depth of the tube. Like doc said. 
 

Are you asking because you are looking to change out your stock? Are you having functional issues? What is driving this conversation?

[98Z5V]

3 hours ago, edgecrusher said:

What is driving this conversation?

Raping a 6-yeal old thread is what's driving this conversation, right now... NecroPosting...  I'm just sayin'...

[kenk]

On 7/19/2020 at 6:01 PM, edgecrusher said:

I really think he’s confusing the difference between the two and what it pertains to. It’s for your choice of stock and fitment there of. They make stocks for both sizes and you’ll want to choose the correct size for your extension. 
 

it does not have anything to do with the buffer or spring. That has to correlate with the internal depth of the receiver extension (aka buffer tube). Buffer and spring size matter depending on the depth of the tube. Like doc said. 
 

Are you asking because you are looking to change out your stock? Are you having functional issues? What is driving this conversation?

having to replace the extension (buffer tube) because of excessive scarring on the threads from a buddy "showing me how" to use an armorers wrench that was too small.... I do fully understand the difference in stock sizes though, milspec on milspec tube com on com, unless you have a shim you want to try, easy enough. but 98z5v answered my question perfectly, it was on weather the slight thread difference would make any functional difference when exchanging tubes, thank you guys.

 

[kenk]

On 7/19/2020 at 1:30 AM, 98Z5V said:

That's not something to worry about, and the difference in that drawing is there because the milspec uses rolled threads, and the commercial uses cut threads.

The only way to tell the difference isn't by looking at them - it's by measuring the O.D.  Mil=spec is 1.14" OD, and commercial is 1.17" OD. 

That will tell you right there what you have, right away.  Digital vernier caliper is the fastest way to find out.  Don't measure threads, and waste your time - measure the smooth part.  You'll know right away.

This should break it down pretty easy for you:

thank you for that, totally explains my worry there. I knew the difference was very slight, and that the kind of thread made up for most of it, but the picture takes away all doubt. You guys are great, thanks again.

[edgecrusher]

Awesome, let us know how it works out

[392heminut]

On 7/18/2020 at 1:03 PM, kenk said:

 I guess I need to measure it with calipers to be certain, the reason I say they are exactly the same is the buffer tube nut off of mine fit the tube and threads perfectly for the "ar 15 mill spec" one.

The nuts all fit the different tubes. You need to measure the smooth part of the tube with caliper to get a true diameter.

1.148 = mil-spec diameter

1.168 = commercial diameter