CMMG MK4 Pistol Lower Receiver 5.56 NATO

Description

The CMMG MK4 Pistol Lower Receiver 5.56 NATO is the foundation for building a high-quality, short-barreled AR-style pistol. It’s a stripped lower receiver, meaning it’s the serialized “firearm” part you’ll build upon with your chosen upper, trigger, and stock/brace components. This specific model is designed for 5.56/.223 builds and is the starting point for a compact, powerful personal defense or range tool. I’ve built several pistols on this platform, and its consistency is what keeps me coming back.

Here’s what makes the CMMG MK4 stand out:

• Forged 7075-T6 Aluminum: This is the same mil-spec material used in duty rifles, offering a great balance of strength and manageable weight for a pistol build.
• Mil-Spec Compatibility: This is the key. It’s designed to work with the vast majority of standard AR-15 upper receivers, parts kits, and triggers. That said, pairing it with a CMMG Resolute upper is a guaranteed seamless fit.
• Anodized Finish: Provides a durable, corrosion-resistant surface that holds up to handling and the elements.
• Pistol-Designated: It’s transferred and registered as a pistol receiver, which is legally necessary for building a firearm with a barrel under 16″ without NFA paperwork.

Specifications

Caliber	5.56 NATO / .223 Remington
Material	Forged 7075-T6 Aluminum
Finish	Type III Hardcoat Anodized
Fire Control Pocket	Mil-Spec
Trigger Guard	Integrated
Receiver Extension	Mil-Spec Buffer Tube Threads

Who It’s For: This lower is perfect for the builder who wants a no-compromise starting point for a serious AR pistol. You’re not looking for a budget polymer option; you want the real-deal foundation that will reliably pair with a quality upper like a BCM or Aero Precision. It’s for someone who understands that the lower is the heart of the firearm and is willing to invest in proven quality. At $265.92, it sits in that sweet spot between basic and boutique.

Pros & Cons
Pros: The forged construction is incredibly tough. The mil-spec dimensions mean virtually everything is drop-in ready—no gunsmithing required for standard parts. CMMG’s quality control is excellent, so you get a clean, well-machined piece. It’s a trusted platform I’ve personally run hard without a single receiver-related issue.

Cons: Look, it’s a stripped receiver. The price is just the entry fee—you’ll spend significantly more to complete the firearm. It’s also just metal; there are no fancy ambidextrous controls or custom engraving at this price point. You’re paying for a superior blank canvas.

Frequently Asked Questions

Q: Is this a complete firearm?

A: No. This is a stripped lower receiver, which is the serialized part legally considered the “firearm.” It requires additional parts (upper receiver assembly, buffer kit, fire control group, pistol brace/stock) to become a functioning weapon. An FFL transfer is required for purchase.

Q: What parts do I need to complete it?

A: You’ll need a Lower Parts Kit (LPK), a pistol buffer tube kit, a fire control group (trigger), and a pistol brace or stock. Then, you’ll mate it with a complete 5.56 upper receiver assembly. All these components are widely available and install with basic tools.

Q: Can I use this to build a rifle?

A: Legally, yes. Since it’s transferred as a “receiver,” you can build it into either a pistol or a rifle. However, if you first build it as a rifle, you cannot later convert it to a pistol. The reverse (pistol to rifle) is generally permissible. Always confirm current ATF regulations.

If you’re ready to start a build with a foundation that won’t let you down, this CMMG MK4 lower is available at firearms-missouri.com for Missouri residents and nationwide. Check local laws for pistol build compliance.