B5 SRK EXTENSION KIT BLK

Description

The Sierra K Receiver Extension Kit is the dedicated mounting solution for the B5 Systems Sierra K Buttstock, designed for shooters running a shorter-than-standard AR-15 platform who need a buffer system matched to that reduced envelope. Because a standard carbine receiver extension is too long to allow the Sierra K to function correctly, this kit replaces the entire rear assembly rather than asking you to adapt mismatched components.

At the core of the kit is a reduced-length, mil-spec diameter receiver extension machined from 7075 aluminum — a alloy chosen for its strength-to-weight ratio in demanding applications. The five-position adjustment is retained, so you still get a meaningful range of length-of-pull settings despite the shortened tube. The included short heavy buffer weighs 3.5 oz, which is heavier than a standard carbine buffer; that additional mass slows the bolt carrier’s rearward travel, which matters on a short-stroke system where timing is tighter. The extra-power spring works in tandem with that buffer weight to ensure reliable cycling and positive bolt lockup. A castle nut and quick-disconnect end plate round out the package, so everything needed to swap out a standard buffer assembly is included in one box.

All components are built to mil-spec materials and finishes and manufactured in the United States. If you are already running or planning to run the Sierra K stock, this kit is not optional — it is the system the stock was engineered around.