how do i know if i have a mil-spec or commercial buffer tube on my CMMG AR-15 for a replacement stock?

The milspec tube is a little narrower.

All you need to do is measure the outside diameter of the buffer tube across the width of the tube. Commerical tubes are 1.170 and mil-spec tubes are only 1.140. Surprisingly, three-hundredth of an inch make a world of difference in the stock fit!!

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http://thefiringline.com/forums/showthread.php?t=4...

Firstly, I might have misunderstood your question, but there is no commercial or milpec A2 tubes - they are their own thing A2 tube = A2 tube. It is longer and accepts the A2 stock and the magpul PRS stock and a couple others, but not the collapsable ones. If you want a collapsable stock, you can get either a milspec or commercial tube/reciever extension, but as you said, yes the commercial stocks with rattle on a milspec tube. You may be able to shim it, but most places offer either commercial or milspec so it would be pointless to mis-match them deliberately. The higher tension springs offer a cheap way to tune an out of balance gun. On a carbine or mid length gas system, they can help a lot, but in your case they are more likely to cause problems than help them - your rifle length gas system runs sweet on a regular spring as they were designed to be together. However, for the noises, you can follow the advice from Big Hammer (and I am going to try that myself - I had not heard of that before). Or you can buy an oversized, polished spring that will take out the TWANG and the scraping noises. There is one in the link below, but you can get them cheaper from Brownells if you sign up. I have 3 of them and they are worth the $20.