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Posted: 10/4/2012 8:47:24 PM EDT
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Built up a new CMMG lower, used a PSA Classic Premium M4 Stock Kit Black.
After I shouldered this gun for the first time I knew something was wrong but at first didn't know what. Later I realized my natural check weld was too close to the sights / scope. Tonight I broke out the tap measure and pulled the stocks off and sat the tubes side by side. Do mil spec buffer tubes come in different lengths? CMMG tube = 7 1/4" PSA tube = 6 1/4" 
I apologize for the tape position it kept moving and was camera shy. 
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http://www.ar15fornoobs.com/mil-spec-vs-commercial-spec-buffer-tubes Look here, hope this helps. |
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You can't actually measure it properly when it's installed, since you're showing runout lengths past the stock nut in your photos.
The exterior length is actually not important to the function, as the "seemingly long" tube could have a fat solid backside, and the correct recoil length on the inside. My guess since you didn't label yours is that the seemingly short one is the commercial variant - with a fatter tube, thinner back/end area, and cut threads. It may be perfectly fine operationally if the interior dimensions are the same as the milspec tube. Try running your tape measure down the inside of the empty extensions and comparing that depth number. There is only one milspec length for a carbine receiver extension, as shown in the attachment above my reply. The exterior dimensions and means of forming the thread can vary on non-milspec tubes. Milspec requires rolled threads, not the cheaper cut threads. |
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Well crap, they are both 7" exactly when measured from the inside, for gun function that may be the only important factor.
But for length of pull it's a big deal. It's about 3/4" difference between the last notch on the CMMG tube vs. the PSA tube. This is looking like my Mil-Spec CMMG M4LEP22 shipped with a commercial tube. (longer, slant back) In one way I'm pissed because this rifle was supposed to be Mil-Spec, on the other I'm glad to have the longer length of pull. 
CMMG on bottom 
CMMG on left 
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