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4/25/2017 7:42:44 PM
Posted: 2/17/2010 4:20:07 PM EDT
Hi all, while reading I see there are Mil spec and commercial buttstocks. What is the difference, and advantages of one over the other?

Thanks
Link Posted: 2/17/2010 4:22:24 PM EDT
[Last Edit: 2/17/2010 4:23:11 PM EDT by ziggiey]
Different tube size, mill spec is thought to be stronger and has more available stocks. Sopmod and some others are only available in mill spec size.
Link Posted: 2/17/2010 4:23:26 PM EDT
Mil-spec are the best ones to have. You have more options for stocks. Commercial stocks are used by Rock River and a couple of others. There isn't any advantage in using commercial stocks, so I am not sure why they still do.

There is a size difference though. You can't put a mil-spec stock on a commerical buffer tube as it is too large for the stock to fit.

Definitely get a mil-spec tube.
Link Posted: 2/17/2010 4:24:20 PM EDT
Here:
Link Posted: 2/17/2010 4:28:28 PM EDT
[Last Edit: 2/17/2010 4:32:21 PM EDT by Captains1911]
mil-spec receiver extension have extruded threads that fully engage the threads in the lower receiver. Commercial extension tube threads are of lesser diameter and do not fully engage the lower threads, it is a manufacturing shortcut that leads to a weaker extension to receiver connection. The reason why the stocks are different is because the tube diameters of the extensions are slightly different between the two.
Link Posted: 2/17/2010 5:22:24 PM EDT
Man this site is great!!

Thanks so much for the explanations everyone! I will look for a Mil spec buttstock then

Thanks again!
Link Posted: 2/17/2010 5:26:20 PM EDT
Now that we are talking about it, why in the hell are there two different tube options? Why not just have one and be done with it?
Link Posted: 2/17/2010 5:32:19 PM EDT
[Last Edit: 2/17/2010 5:32:50 PM EDT by Captains1911]
Originally Posted By blackfly53:
Now that we are talking about it, why in the hell are there two different tube options? Why not just have one and be done with it?


like I said, it's cheaper to manufacture the commercial receiver extension because the outside thread and tube diameters are the same.
Link Posted: 2/17/2010 5:54:00 PM EDT
You'll find the "commercial" size receiver extension on DPMS, RRA, and other AR manufacturers although they do offer "mil-spec" as an optional upgrade like they do with chrome lined barrels.

Colt, S&W, and many other manufacturers offer the "mil-spec" diameter as standard.

Link Posted: 2/17/2010 5:54:38 PM EDT
Ok, so I have a DPMS stripped lower, will that lower accept either Mil spec or commercial? I would like to stick with the Mil spec(based on your recommendations here) if it will work with my DPMS lower.

Thanks
Link Posted: 2/17/2010 5:56:28 PM EDT
[Last Edit: 2/17/2010 5:57:44 PM EDT by Captains1911]
Originally Posted By spleify:
Ok, so I have a DPMS stripped lower, will that lower accept either Mil spec or commercial? I would like to stick with the Mil spec(based on your recommendations here) if it will work with my DPMS lower.

Thanks


Yes, all lowers will accept both mil-spec and commercial extensions (assuming of course that the lowers are within spec).
Link Posted: 2/17/2010 6:03:50 PM EDT

Yes, all lowers will accept both mil-spec and commercial extensions (assuming of course that the lowers are within spec).


Is it common for lowers to be out of spec? Is it common for the DPMS to be out of spec? Are the DPMS lowers any good?

Thanks for the reply!


Link Posted: 2/17/2010 6:41:56 PM EDT
Originally Posted By Captains1911:
Originally Posted By blackfly53:
Now that we are talking about it, why in the hell are there two different tube options? Why not just have one and be done with it?


like I said, it's cheaper to manufacture the commercial receiver extension because the outside thread and tube diameters are the same.


I beleive those are great reasons for the difference, but is that that the official reason or just your opinion?

Link Posted: 2/17/2010 7:02:58 PM EDT
Originally Posted By Captains1911:
mil-spec receiver extension have extruded threads that fully engage the threads in the lower receiver. Commercial extension tube threads are of lesser diameter and do not fully engage the lower threads, it is a manufacturing shortcut that leads to a weaker extension to receiver connection. The reason why the stocks are different is because the tube diameters of the extensions are slightly different between the two.


Yup I have seen a couple of those generic commercial tubes with the undersized threads at the gun show right next to the generic FCG parts.

Otherwise all of the commercial tubes I have seen since the late '70s have had full diameter threads. I have seen one of the slanted back tubes ( don't remember the brand ) the rest have been normal.

About the only advantage to the commercial stock that I can think of is that they have been on the market far longer and in greater numbers than milspec units so the likely hood of picking up a used one or parts should be greater.

Either way be your own final Q.C. inspector and look your parts over as carefully as you would a gun.
Link Posted: 2/17/2010 7:07:51 PM EDT
Originally Posted By marmilia:
Here:
http://i46.tinypic.com/2hcf1ba.jpg


There can be 6 position mil spec ones right?
Link Posted: 2/17/2010 7:25:51 PM EDT
Early milspec CAR tubes had only two positions.
Three four and six positions were Civilian inventions later adopted by the Mil services.
Link Posted: 2/17/2010 7:30:34 PM EDT
[Last Edit: 2/17/2010 7:39:36 PM EDT by rjhoop1]
yea they make six position mil-spec!!!!
Link Posted: 2/17/2010 7:37:17 PM EDT
Originally Posted By B44T:
Originally Posted By Captains1911:
mil-spec receiver extension have extruded threads that fully engage the threads in the lower receiver. Commercial extension tube threads are of lesser diameter and do not fully engage the lower threads, it is a manufacturing shortcut that leads to a weaker extension to receiver connection. The reason why the stocks are different is because the tube diameters of the extensions are slightly different between the two.


Yup I have seen a couple of those generic commercial tubes with the undersized threads at the gun show right next to the generic FCG parts.

Otherwise all of the commercial tubes I have seen since the late '70s have had full diameter threads. I have seen one of the slanted back tubes ( don't remember the brand ) the rest have been normal.

About the only advantage to the commercial stock that I can think of is that they have been on the market far longer and in greater numbers than milspec units so the likely hood of picking up a used one or parts should be greater.

Either way be your own final Q.C. inspector and look your parts over as carefully as you would a gun.


I know im a newbe on here but come on!!!!!!!!!But hey i guess this is just like every other sight, you have to hope some one says something intellegent before you go to the bank
Link Posted: 2/18/2010 4:36:21 AM EDT
Originally Posted By blackfly53:
Originally Posted By Captains1911:
Originally Posted By blackfly53:
Now that we are talking about it, why in the hell are there two different tube options? Why not just have one and be done with it?


like I said, it's cheaper to manufacture the commercial receiver extension because the outside thread and tube diameters are the same.


I beleive those are great reasons for the difference, but is that that the official reason or just your opinion?



It's fact, not opinion.
Link Posted: 2/18/2010 5:32:13 AM EDT

Originally Posted By spleify:

Yes, all lowers will accept both mil-spec and commercial extensions (assuming of course that the lowers are within spec).


Is it common for lowers to be out of spec? Is it common for the DPMS to be out of spec? Are the DPMS lowers any good?

Thanks for the reply!


DPMS lowers are good-to-go, I have always found them to be economical, high quality, and in spec.

I have used them for several builds. I always use components that are higher grade than what comes standard in an "off the shelf" DPMS product. Your mil-spec receiver extension and stock will be just fine on a DPMS receiver.

Link Posted: 2/18/2010 6:01:04 AM EDT
Good to hear! Thanks for the vote of confidence.

You guys are great, thanks for all the help.

Spleify
Link Posted: 2/18/2010 6:33:02 AM EDT
So the mil-spec tube itself isn't necessarily stronger than the commercial, but the connection to the receiver is. Am I following that right?
Link Posted: 2/18/2010 6:53:14 AM EDT
Originally Posted By Grey_Man:
So the mil-spec tube itself isn't necessarily stronger than the commercial, but the connection to the receiver is. Am I following that right?


Exactly. The diameter of the threads on the mil-spec tube are larger than the diameter of the tube itself and they fully engage the threads on the lower receiver. The threads on a commercial tube are of the same diameter as the tube itself and do not fully engage the lower, hence a weaker connection.
Link Posted: 2/18/2010 7:43:17 AM EDT
Link Posted: 2/18/2010 7:46:08 AM EDT
Just out of curiousity...

Everyone keeps saying that milspec tubes have more stock options.

Other than the LMT sopmod, what other stocks don't come in both sizes?
Link Posted: 2/18/2010 7:50:09 AM EDT
[Last Edit: 2/18/2010 7:50:52 AM EDT by Justin-Beard]
Link Posted: 2/18/2010 6:11:15 PM EDT
Originally Posted By Justin-Beard:
Ok, this should answer everyone's questions:

"Mil-spec" and "commercial-spec" for receiver extensions pretty much just relates to sizing these days. A true mil-spec tube will be machined from a solid bar of 7075-T6 alloy and have its threads rolled to a 1-3/16–16 UN thread. Commercial-spec extensions were introduced as a means to reduce costs by going with an extruded tube that has the end cap welded on (usually using a lesser material such as 6061-T6) and the threads cut to the same 1-3/16–16 UN thread. They are also a slight bit longer in most cases (and in general have a slanted back, but not always). The reason why commercial-spec is larger in diameter is that when you cut threads you end up removing material. When you roll threads you are basically squeezing the threads into shape and they end up being larger in diameter than the tube. Therefore to keep the same thread size for your receiver's 1-3/16–16, a commercial-spec tube must be a larger diameter.

Now here's where things get interesting. Many if not most mil-spec extensions are just mil-spec sized and not built to the military specs of using a solid bar of 7075-T6 and rolling the threads. Add to that the fact that every manufacturer uses their own specs for the size of their tubes and you are left with a dizzying variety of carbine receiver extensions to choose from. In most cases, however, the average consumer will be hard pressed to notice any performance differences between extensions and as long as your particular stock fits your particular tube well then there's not much to worry about. Of course finding a combo that actually does fit well together can sometimes be a challenge with the huge variety of manufacturers producing extensions and/or stocks. Unfortunately there are very little standards in the AR15 world...



Thank you for that explanation. I have always wondered...

Link Posted: 2/18/2010 8:42:12 PM EDT
Yes, thanks for the great explanation.
Link Posted: 4/6/2010 10:46:02 AM EDT
Wow. This has been the most helpful thread I've red today. I've always wondered the difference. I have been looking to upgrade the stock that was on my RRA lower half which has a commercial tube and stock. I want the Vltor Emod and noticed that even though you can get it in commercial, it's more widely sold in mil-spec. Judging from what I've read here, I may run into minor fit problems if I try to match their in house commercial specs with RRA commercial specs. I'm better off getting the the mil-spec and stock kit and replacing the unit as a whole. Right?

....now, to make up my mind between the Vltor Emod and the Magpul UBR......hmmmmm.....
Link Posted: 4/6/2010 10:56:01 AM EDT
[Last Edit: 4/6/2010 10:56:36 AM EDT by Skyyr]
Originally Posted By Justin-Beard:
Originally Posted By Captains1911:
Originally Posted By Grey_Man:
So the mil-spec tube itself isn't necessarily stronger than the commercial, but the connection to the receiver is. Am I following that right?

Exactly. The diameter of the threads on the mil-spec tube are larger than the diameter of the tube itself and they fully engage the threads on the lower receiver. The threads on a commercial tube are of the same diameter as the tube itself and do not fully engage the lower, hence a weaker connection.

Actually the weaker connection would be from the fact that commercial-spec extensions are usually made from a weaker material (6061-T6) and use inferior cut threads versus a true mil-spec extension made from 7075-T6 utilizing stronger rolled threads.



So which is the UBR's entry receiver extension? I mean, obviously the length and diameter may not conform to mil-spec, but what about the threads?
Link Posted: 4/6/2010 1:05:03 PM EDT
[Last Edit: 4/6/2010 1:05:35 PM EDT by Blacksnake]
Originally Posted By Justin-Beard:
Ok, this should answer everyone's questions:

"Mil-spec" and "commercial-spec" for receiver extensions pretty much just relates to sizing these days. A true mil-spec tube will be machined from a solid bar of 7075-T6 alloy and have its threads rolled to a 1-3/16–16 UN thread. Commercial-spec extensions were introduced as a means to reduce costs by going with an extruded tube that has the end cap welded on (usually using a lesser material such as 6061-T6) and the threads cut to the same 1-3/16–16 UN thread. They are also a slight bit longer in most cases (and in general have a slanted back, but not always). The reason why commercial-spec is larger in diameter is that when you cut threads you end up removing material. When you roll threads you are basically squeezing the threads into shape and they end up being larger in diameter than the tube. Therefore to keep the same thread size for your receiver's 1-3/16–16, a commercial-spec tube must be a larger diameter.

Now here's where things get interesting. Many if not most mil-spec extensions are just mil-spec sized and not built to the military specs of using a solid bar of 7075-T6 and rolling the threads. Add to that the fact that every manufacturer uses their own specs for the size of their tubes and you are left with a dizzying variety of carbine receiver extensions to choose from. In most cases, however, the average consumer will be hard pressed to notice any performance differences between extensions and as long as your particular stock fits your particular tube well then there's not much to worry about. Of course finding a combo that actually does fit well together can sometimes be a challenge with the huge variety of manufacturers producing extensions and/or stocks. Unfortunately there are very little standards in the AR15 world...



Thank God for the voice of sanity.

I've never heard of an "extruded thread" before in reference to a threaded part (one of the previous posts before yours).

Glad you showed up to (most tactfully) explain the difference between rolled and cut threads.
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