B5 Systems Sierra K Extension Kit - AR15 Compatible

B5 Systems Sierra K Extension Kit - AR15 Compatible

Optimize your AR-15 platform for ultra-compact maneuvering with the B5 Systems Sierra K Receiver Extension Kit in Black. Purpose-built as the proprietary mounting system for the B5 Systems Sierra K Buttstock, this reduced-length receiver extension setup shortens your firearm's overall footprint without compromising cycling dynamics or functional reliability. Machined from ultra-durable 7075 aluminum to mil-spec tolerances, it provides a rugged, 5-position buffer tube architecture engineered to handle hard-use operational environments.

To ensure smooth bolt carrier group travel and consistent feeding in a shortened tube envelope, the Sierra K Kit includes a specially calibrated 3.5-ounce short heavy buffer paired with an extra-power action spring. The comprehensive package also includes a mil-spec castle nut and a quick-detach (QD) endplate, providing an instant rear sling attachment point out of the box. Proudly made in the USA using mil-spec materials and finishes, this extension kit delivers a complete, professional-grade solution for building a streamlined, compact rifle setup.

Features

Reduced-Length 5-Position 7075 Aluminum Tube:
Machined from high-strength 7075 aluminum with a mil-spec hardcoat anodized black finish, offering 5 adjustment positions in a shortened receiver footprint.

Optimized Short Heavy Buffer (3.5 oz) & Extra Power Spring:
Includes a 3.5-ounce short heavy buffer and an extra-power action spring calibrated to maintain proper bolt velocity and system timing.

Proprietary Sierra K Mounting Interface:
Engineered specifically to serve as the dedicated buffer tube and mounting baseline for the B5 Systems Sierra K Buttstock.

Integrated QD Sling Mounting Endplate:
Ships complete with a heavy-duty QD endplate and castle nut, allowing effortless integration of push-button quick-detach slings.

100% Mil-Spec Materials & Made in the USA:
Built entirely in the United States using military-specification materials and finishes for long-term durability and structural integrity.

Specifications