B5 Systems Sierra K SRK-1660 AR-15 Receiver Extension Kit - Black

The B5 Systems Sierra K SRK-1660 Receiver Extension Kit is purpose-built for compact AR-15 PDW and SBR builds, delivering a shortened stock footprint without sacrificing the strength of a true mil-spec diameter tube. At its core is a reduced-length, 5-position receiver extension machined from 7075 aluminum to mil-spec diameter, giving you a rigid, lightweight platform that pairs cleanly with the rest of your carbine.

The kit comes fully equipped with everything needed to drop onto a standard AR-15 lower: a short heavy buffer weighing 3.5 oz tuned for short-stroke setups, an extra power spring to drive reliability with shorter gas systems, a castle nut, and a QD end plate for sling attachment. The Black finish keeps the kit unobtrusive on most builds, and the 5-position layout gives just enough length-of-pull adjustment for use behind a magnified optic or with body armor.

Engineered from the ground up for compact AR platform builds, the Sierra K Extension Kit provides a purpose-tuned solution for shooters who want PDW-length handling with the proven geometry of a mil-spec receiver extension.