The CMMG Receiver Extension Castle Nut (CMMG part no. 55CA9CB) is a black matte castle nut built for CMMG’s MK4 platform, threading onto the buffer tube to lock the receiver extension against the back of an AR-15 lower receiver. CMMG machines this nut to mil-spec threading, so it works alongside a standard castle nut wrench and mil-spec end plate during a build or a buffer tube swap. Properly torquing this nut down is what keeps a collapsible or fixed stock’s receiver extension from working loose under recoil. It’s a small, inexpensive part worth having on hand for any AR build or repair.

Key Specifications

• Finished in black matte to match most AR-15 lower receivers.
• Built for CMMG’s MK4 product line, sized for AR-15/M4-pattern lowers rather than the larger MK3 AR-10 pattern.
• Secures the receiver extension (buffer tube) against the rear of the lower receiver.
• Cut to mil-spec threading, compatible with standard castle nut wrenches and end plates.
• Works with a standard mil-spec end plate as part of the buffer tube assembly.
• Ships at a light 0.045 lb, adding almost nothing to your order.

Frequently Asked Questions

What does a castle nut do? It threads onto the buffer tube and locks the receiver extension against the rear of the lower receiver, keeping it from working loose under recoil.

Does this fit standard mil-spec buffer tubes? Yes, it’s cut to mil-spec threading and works with standard castle nut wrenches and end plates.

Buy the CMMG MK4 Receiver Extension Castle Nut

A loose castle nut is a quiet failure point that shows up at the worst time, and keeping a correctly torqued CMMG nut on hand is cheap insurance against it. Order it from our CMMG lineup, part of our full selection of AR buffer system parts, from our Lubbock, Texas gun shop, and it ships fast to your door.