This BLACKHAWK! Knoxx M4-Style Collapsible Stock is constructed of high impact, glass-reinforced polymer, and is an excellent way to increase the versatility of an AR-15 carbine due to its ability to instantly adjust the length of pull to fit different users or circumstances. While the stock's design closely follows the lines and general appearance of the US military's M4 Carbine buttstock, several subtle improvements have been integrated to enhance its overall functionality. These include a more substantial comb for a comfortable cheek weld and reduced neck fatigue, a pair of side-mounted, quick detach sling swivel sockets, a recessed stock adjustment latch, and a bottom-mounted, fixed sling swivel stud located at the toe. This stock is compatible with AR-15 and SR-25 / LR-308 type carbines that are equipped with a multi-position receiver extension buffer tube (sold separately).
AR-15 Carbine Receiver Extension Tubes (AKA Buffer Tubes) come in 2 different varieties: Commercial Diameter and Mil-Spec Diameter. To determine which tube size you have, measure the outside diameter of the tube's rounded top section from the sides at its widest point. A Commercial tube will be approximately 1.168" in diameter, while a Mil-Spec tube will be approximately 1.148" in diameter.
In order for your collapsible stock to fit and function as designed, it must be installed on a receiver extension tube of the correct diameter.