CMMG Mk4 AR-15/LR-308 Carbine Receiver Extension Buffer Tube Lock Ring

The CMMG Mk4 AR-15/LR-308 carbine receiver extension buffer tube lock ring is the small but critical castle nut that keeps your buffer tube assembly secure and indexed correctly on your AR platform rifle. Whether you're building out a custom AR-15, upgrading an LR-308, or just need a replacement castle nut, this straightforward part does one job and does it right—no wobble, no drift, no guesswork.

CMMG has been manufacturing AR rifles and components since 2002, and their parts carry the same attention to function and durability you'd expect from a brand that's spent two decades refining the platform. This lock ring handles the interface between your buffer tube and receiver extension with a matte black finish that matches most modern AR builds. It's a wear item you might cycle through a few times over the life of your rifle, especially if you're swapping stocks, tuning gas systems, or experimenting with different buffer weights for precision or recoil management.

Install it with a castle nut wrench and a small punch to stake it properly—it's a five-minute job that ensures your buffer tube stays put under recoil. If you're an AR builder, gunsmith, or just someone who maintains their own rifles, having a spare on the bench makes sense. For hunters, sport shooters, or anyone running a modern carbine regularly, replacing a worn castle nut is preventative maintenance worth doing right.

Key Features

• Castle nut design for secure buffer tube retention on AR-15 and LR-308 platforms
• Matte black finish compatible with standard AR builds
• Fits CMMG Mk4 receiver extensions and standard AR-pattern buffer tubes
• Designed and built by CMMG, manufacturer of AR rifles and components since 2002

Specifications

Brand	CMMG
Product Type	Receiver Extension Buffer Tube Lock Ring / Castle Nut
Compatibility	AR-15 and LR-308 Mk4 receiver extensions
Finish	Matte Black

Frequently Asked Questions

Will this castle nut fit my AR-15 or AR-10?

The CMMG Mk4 lock ring is designed for CMMG Mk4 receiver extensions and is compatible with AR-15 and LR-308 platforms. It fits standard AR-pattern buffer tubes. If you're unsure whether your receiver extension is a CMMG Mk4 model, compare the thread pattern and diameter of your buffer tube before ordering.

Do I need any special tools to install this?

You'll need a castle nut wrench (also called a receiver extension wrench) to install it, and a punch or staking tool to properly stake the nut to the receiver. Both are common tools in any AR builder's kit and are widely available.

Is this a replacement part or does it come as part of a kit?

This is the castle nut alone—a replacement part. It does not include the buffer tube, buffer, or spring; those are sold separately. It's ideal for repair, upgrading existing rifles, or stocking your parts bin for future builds.

What's the difference between a castle nut and other buffer tube fasteners?

A castle nut is a slotted nut designed to be staked (bent slightly through slots) to lock onto the receiver and prevent rotation or loosening from recoil. It ensures your buffer tube assembly stays indexed and secure, which is critical for consistent function and reliability.