ERGO Castle Nut — Steel, Black, for AR-Platform (4971B | UPC 810049123000)
Lock your receiver extension like a pro. The ERGO Castle Nut in steel with a black protective finish (model 4971B) is a duty-ready upgrade for AR-platform builds and retrofits. Engineered for precise fitment and long-term durability, this castle nut secures your pistol buffer tube or carbine receiver extension so your stock or brace stays tight through recoil, sling tension, and hard training.
Why Choose the ERGO Castle Nut
Durable Steel Construction: Heat-treated steel resists deformation during torque, staking, and extended use—far tougher than soft, budget alternatives.
Black Protective Finish: Low-glare black coating adds corrosion resistance and blends seamlessly with modern receivers and end plates.
Correct Thread Spec: Machined for the standard AR-15/AR-pattern receiver extension interface to ensure proper engagement and smooth torque application.
Staking-Friendly Design: Clean staking flats let you mechanically lock the nut after torque so it won’t loosen under recoil or sling load.
Build & Retrofit Ready: Ideal for completing new lowers or upgrading older carbines with a reliable retention solution.
What It Does for Your Build
Prevents Stock/Brace Wobble: Keeps the receiver extension locked in place for a rock-solid interface with the lower receiver.
Maintains Zeroed Ergonomics: Once torqued and staked, your length-of-pull and sling geometry remain consistent—no creeping under stress.
Supports Reliable Cycling: Properly secured extensions help maintain buffer alignment for smooth, repeatable operation.
Ideal Uses
Duty/Defensive Carbines: Trustworthy retention for rifles that see rough handling and frequent sling use.
Competition & Training Rigs: Repeated transitions and high round counts demand a nut that won’t back off.
Retrofits & Refreshes: Replace worn or marred castle nuts to restore factory-tight lock-up.
Installation Overview (Armorer-Style Best Practices)
Verify Clear & Safe: Unload the firearm and separate upper from lower.
Prep the Hardware: Slide the end plate onto the receiver extension; thread the ERGO Castle Nut with the staking notches facing rearward.
Set the Extension: Thread into the lower until the buffer retainer and spring are properly captured—don’t crush the retainer.
Align the End Plate: Seat the takedown detent spring and press the end plate flush against the lower.
Torque: Using a quality armorer’s wrench, torque the castle nut to spec (commonly 38–42 ft-lb—follow your agency or manufacturer guidance).
Stake: With a center punch, displace metal from the end plate into one or two castle-nut notches to mechanically lock the assembly.
Function Check: Ensure takedown pin movement, safety function, and buffer retainer operation are normal.
Tip: Apply a light film of oil to threads before torque; avoid threadlocker when you’ll be staking (mechanical staking is preferred for duty guns).
Key Features & Specs (At a Glance)
Product: ERGO Castle Nut
Material: Steel (staking-friendly, duty grade)
Finish/Color: Black low-glare protective finish
Fitment: AR-Platform (AR-15/AR-pattern) receiver extensions
Use: Secures buffer tube/receiver extension with end plate
Model / MPN: 4971B
UPC: 810049123000
Note: End plate, receiver extension (buffer tube), buffer, spring, and castle-nut wrench sold separately unless otherwise noted.
Compatibility & Pairing
End Plates: Works with standard end plates, including ambidextrous sling and QD styles.
Receiver Extensions: Compatible with mil-spec diameter carbine and pistol extensions (thread spec dependent).
Tools: Use a quality armorer’s wrench with solid engagement lugs to avoid marring.
Frequently Asked Questions
Q: Is this compatible with AR-10/.308 builds?
A: Many .308-pattern lowers share the same castle-nut threading, but receiver extension lengths and buffer systems differ. Verify your platform’s spec before installation.
Q: Do I need threadlocker?
A: For duty or hard-use rifles, staking the nut into the end plate is the preferred method. Most armorers skip threadlocker when staking correctly.
Q: Mil-spec or commercial tube?
A: The ERGO castle nut secures the threaded interface, which is common; tube diameter (mil-spec vs. commercial) affects the stock fit, not the nut itself.
Q: Can I reuse my old end plate?
A: Yes—if it’s undamaged. If previously staked, consider replacing the end plate for clean, secure staking.
Maintenance & Care
Periodic Inspection: Check the staked areas during cleaning. If the nut shows movement or staking wear, re-stake or replace the end plate.
Corrosion Prevention: Wipe with a light protectant after exposure to sweat, rain, or salt.
SEO Highlights (Naturally Integrated)
ERGO castle nut steel black, AR-15/AR-platform castle nut 4971B, secure receiver extension/buffer tube, staking-friendly end plate interface, armorer-grade AR hardware, upgrade AR carbine pistol build, UPC 810049123000.
Bottom Line
The ERGO Castle Nut (4971B) is a small part that makes a big difference. With steel strength, a black protective finish, and staking-ready notches, it’s the reliable way to lock down your receiver extension for a solid, long-lasting AR build.
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