CMMG Receiver Extension Nut (Castle Nut) — Fits AR-15 Buffer Tubes, Matte Black — 55CA9CB (UPC 815835017426)

Lock your receiver extension down with confidence. The CMMG Receiver Extension Nut, commonly called a castle nut, is a rugged, mil-spec–style fastener designed to secure your AR-15 buffer tube to the lower receiver end plate. Precision-fit threads, a durable matte black finish, and CMMG’s proven quality make this small but critical component a smart choice for new builds, refreshes, and duty-ready rifles. Model 55CA9CB, UPC 815835017426.

Why Choose the CMMG Castle Nut

• Mil-spec–style dimensions for broad compatibility with AR-15/M4 pattern lower receivers and carbine receiver extensions.

• Tough steel construction that resists deformation during torque and staking.

• Matte black finish for a low-glare, uniform look that pairs with most end plates and buffer tubes.

• Clean, precise threading for smooth installation and secure lock-up.

• CMMG reliability trusted by armorers, agencies, competitors, and enthusiasts.

Purpose-Built for Solid Lock-Up

Your castle nut does more than finish the job—it maintains alignment of the receiver extension, keeps the end plate properly tensioned, and protects the rear takedown detent spring from shifting. The CMMG 55CA9CB is engineered to handle repeated recoil cycles and routine maintenance without working loose when properly torqued (and staked if required by your protocol).

Compatibility

• Platform: AR-15/M4 pattern lowers (MK4/standard AR).

• Receiver Extension: Carbine-length buffer tubes (mil-spec outer diameter).

• Works with: Standard low-profile end plates and most sling/end-plate accessories that follow mil-spec geometry.

Note: Not intended for rifle-length receiver extensions. If using proprietary buffer tubes or specialized end plates, verify interface dimensions.

Ideal Use Cases

• New Lower Builds: Complete your assembly with a dependable, mil-spec–style castle nut.

• Upgrades & Repairs: Replace marred or worn nuts during stock changes or buffer system maintenance.

• Hard-Use Carbines: Secure your extension for high round counts and frequent field stripping.

Installation Best Practices (General Guidance)

1. Unload and clear the firearm before any work.

2. Dry-fit the receiver extension, end plate, and castle nut; confirm the rear takedown detent spring is properly captured.

3. Torque the castle nut to a specification consistent with your armorer’s manual.

4. Stake one or two notches (if policy requires) to prevent rotation over time.

5. Perform a function check to ensure proper safety, takedown, and buffer travel.

Always follow manufacturer instructions and current armorer standards. If unsure, consult a qualified gunsmith.

Clean, Low-Profile Aesthetics

The matte black finish reduces glare and blends seamlessly with anodized receivers, nitrided buffer tubes, and black oxide end plates. The low-profile notches accept common armorer wrenches for solid purchase without excessive bulk or snag points.

Technical Details

• Product: CMMG Receiver Extension Nut / Castle Nut

• Model / SKU: 55CA9CB

• UPC: 815835017426

• Platform: AR-15 / M4 (carbine receiver extension)

• Material: Steel

• Finish: Matte Black

• Interface: Mil-spec–style threads for standard carbine buffer tubes

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The Bottom Line

Small parts make a big difference. The CMMG Receiver Extension Nut (55CA9CB) delivers the precision, durability, and finish you want for a professional-grade lower receiver assembly—whether you’re building from scratch or replacing a tired component.

The End