There are two types of carbine receiver extensions (buffer tubes): Mil-Spec and Commercial. The Mil-Spec tubes are smaller than the Commercial tubes so you will need to determine which extension is on your particular rifle before buying accessories that fit on the receiver extension. Here is how to tell them apart.
The Mil-Spec tube cross-section measures 1.145 to 1.150″ across the sides. The tube is 7.25″ long and generally has a flat back. This is the smaller of the two types of receiver extension.
Commercial tube cross-sections measure 1.163 to 1.173″ across the sides. The tube is 7.80″ long and usually have a slanted back, with the top of the tube being slightly longer than the bottom where the guide rail is located. This is the larger of the two types of receiver extension.
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