AR15 Milspec receiver extension tube vs. Commercial receiver extension tube

[Dick Proenneke]

Just thought I'd post this diagram as I have been PM'd a few questions as of late about the difference;

 

[USP]

some manufacturers make only Milspec tubes others commonly make commercial tubes. The stock hole pattern in the pictures isnt an indicator either (not that you said it was) as some tubes can have 4 or 6 and still be Milspec.

 

[CdnCombatMedic]

I am also under the understanding that not all comm tubes have a slant also.

 

[Undergiver]

I am also under the understanding that not all comm tubes have a slant also.

4 position commerical does not have the slant.

 

[Winz]

Really the best way to tell is the tube diameter.

 

[RedDuc]

A commercial buttstock fits on milspec tube with a bit of wiggle.
But a milspec buttstock will not fit on commercial tube.