The Luth-AR Carbine Lock Ring / Castle Nut (brand part number CS-02) is a replacement castle nut built to retain and torque a carbine buffer tube against an AR-15 lower receiver. This small but essential part threads onto the receiver extension and, combined with the receiver end plate, locks the buffer tube securely in place so it cannot back out under recoil. It is a simple, inexpensive way to replace a lost, damaged, or worn castle nut without buying an entire buffer tube kit. Builders assembling a lower from individual parts, or repairing an existing carbine stock setup, will find this an easy drop-in fix.

Key Specifications

• Functions as a carbine lock ring and castle nut for AR-15 buffer tube installation.
• Secures the receiver extension (buffer tube) against the lower receiver and end plate.
• Sized for standard AR-15 carbine buffer tube threading.
• Sold as an individual replacement part, not bundled into a full buffer tube kit.

Frequently Asked Questions

What does a castle nut do? It threads onto the buffer tube and locks against the receiver end plate to keep the buffer tube from backing out of the lower receiver under recoil.

Do I need a wrench to install this? A castle nut wrench or armorer’s tool is the standard way to properly torque a castle nut during installation.

Will this fit any AR-15 buffer tube? It is sized for standard AR-15 carbine buffer tube threading.

Buy the Luth-AR Carbine Lock Ring / Castle Nut

Order the Luth-AR carbine castle nut from our Lubbock, Texas gun shop for fast, insured shipping. See more Luth-AR gun accessories and parts for your build.