The B5 Systems Sierra K Receiver Extension Kit (brand part no. SRK-1660) is the proprietary mounting system built specifically to run the Sierra K buttstock. B5 Systems machines the reduced-length, mil-spec diameter receiver extension from 7075 aluminum and pairs it with a short 3.5-ounce heavy buffer and an extra-power spring so the Sierra K stock cycles correctly. The kit rounds out with a castle nut and quick-disconnect end plate, giving a builder everything needed to mount the stock without hunting down individual parts.

Key Specifications

• Purpose-built mounting system for the B5 Systems Sierra K buttstock.
• Reduced-length, mil-spec diameter receiver extension.
• Machined from 7075 aluminum for strength and durability.
• 5-position receiver extension for adjustable length-of-pull.
• Includes a short, 3.5-ounce heavy buffer matched to the stock.
• Includes an extra power spring for reliable cycling.
• Includes a castle nut and quick-disconnect end plate for a complete installation.

Frequently Asked Questions

What stock is this kit designed for? The B5 Systems Sierra K buttstock.

What does the kit include? A 7075 aluminum receiver extension, short heavy buffer, extra power spring, castle nut, and QD end plate.

Is the buttstock included? No, this kit is the mounting hardware only.

Buy the B5 Systems Sierra K Receiver Extension Kit

Get everything needed to mount a Sierra K stock in one matched kit. Order it from our AR buffer system selection at our Lubbock, Texas gun shop for fast, direct shipping.