Ergo 4971B Castle Nut Steel Black for AR-Platform Rifles

The Ergo 4971B Castle Nut in steel black is a purpose-built replacement castle nut for AR-platform rifles. If you're assembling an AR, upgrading a stock, or doing maintenance on your lower receiver, a quality castle nut matters more than you might think—it's the component that securely fastens your buttstock to the buffer tube, and it needs to stay put under recoil. The Ergo 4971B delivers solid construction in a finish that matches modern black AR furniture.

Castle nuts come in a few flavors, and the 4971B is built for reliability on the job. Whether you're running a milspec buffer tube setup or need a replacement after years of field use, this steel nut provides the secure, repeatable clamping you need. It's a straightforward, no-frills part that does exactly what it's supposed to do—hold your stock in place with zero wiggle.

For any AR builder or shooter looking to refresh lower-receiver hardware or complete a stock installation, the Ergo 4971B is a solid choice that won't let you down.

Key Features

• Steel construction for durability and secure fastening
• Black finish matches modern AR lower receivers and stock hardware
• Designed for AR-platform rifles with milspec buffer tubes
• Straightforward installation on lower receiver assembly

Frequently Asked Questions

What AR rifles is the Ergo 4971B compatible with?

The 4971B is designed for AR-platform rifles with standard milspec buffer tubes. It works with most modern AR-15 and AR-10 style builds that use conventional lower receivers.

Do I need to replace my castle nut, or is it just an upgrade?

The 4971B works as either a replacement part if your existing castle nut is damaged or worn, or as a swap if you're upgrading your stock assembly and want matching hardware.

What tools do I need to install a castle nut?

Installation typically requires a castle nut wrench (sometimes called a stock wrench) and an armorer's wrench to hold the lower receiver. Specific tool requirements depend on your build, so consult your lower or stock manufacturer's instructions.