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AR15.COM
2/25/2008 9:13:17 PM EDT
How do I know if I need to order a mil-spec or a comercial CTR?
2/26/2008 3:52:28 AM EDT
[#1]
Did a rubber butt pad come with your stock attached?  If so that is a commercial.  otherwise it is a mil-spec.
2/26/2008 5:29:14 AM EDT
[#2]
OP sounds like they don't have the stock yet.

Go to the Magpul website and look at the CTR page... it explains.

From what I've read, the easy way to tell is to look at the threads on your receiver extension (aka buffer tube).  If the threads are flush with the rest of the tube, it's a commercial tube.  If the threads have a larger outside diameter than the smooth portion of the tube (not including the square part at the bottom), then you have a mil-spec tube.
Another way to tell is if you know the maker of the tube... RRA, Bushmaster, DPMS = commercial.  Stag, Colt = mil-spec.  There are several other explainations to the question on this site also.
2/26/2008 7:48:11 AM EDT
[#3]
Also the buffer tube on a commercial is angled at the bask while a Mil-spec is 90 degrees.
2/26/2008 12:51:31 PM EDT
[#4]
Thank for the info. Looks like I need a mil-spec.