Description
The CMMG MK4 K 5.56x45mm 12.5″ Medium Weight Multi-Role Barrel A2 Compensator RKM11 KeyMod Hand Guard Magpul MOE Pistol Grip and Stock Black 30rd – All NFA Rules Apply is a factory-built, short-barreled rifle (SBR) configuration AR-15. It’s a compact, potent 5.56mm platform designed for maneuverability in tight spaces, built with quality components like a CMMG medium-weight barrel and Magpul furniture, requiring an approved ATF Form 4 for transfer.
Key Features
- 12.5″ Medium-Profile Barrel: Balances weight and heat dissipation for a compact package that maintains ballistic effectiveness.
- RKM11 KeyMod Handguard: Provides a slim, lightweight mounting platform for lights, grips, and other mission-critical accessories.
- Magpul MOE Stock & Grip: Delivers proven ergonomics and a solid cheek weld for immediate control and comfort.
- A2 Compensator: Helps manage muzzle rise, keeping you on target for faster follow-up shots.
- Complete Upper Assembly: This rifle is drop-in ready and mates with any mil-spec AR-15 lower receiver, simplifying the NFA process.
Specifications & Details
| Caliber | 5.56x45mm NATO |
| Barrel Length | 12.5 inches |
| Handguard | CMMG RKM11 KeyMod, 11 inches |
| Stock & Grip | Magpul MOE |
| Muzzle Device | A2 Compensator |
| Magazine Capacity | 30 Rounds |
When it comes to home defense or vehicle-based operations, length matters. The 12.5-inch barrel shaves critical inches off the overall package compared to a 16-inch rifle, making it far easier to navigate hallways or deploy from a confined space. The medium-weight barrel construction handles sustained fire better than a pencil profile, a practical consideration during extended training sessions. With a price point of $878.04, this CMMG build offers a direct path to a capable SBR without the cost and guesswork of a custom build.
Who This Rifle Is For
This CMMG MK4 K is for the shooter who understands the tactical advantage of a compact rifle and is prepared to navigate the National Firearms Act process. It’s ideal for a designated home defense weapon where maneuverability is paramount, or as a dedicated patrol carbine for private security. It also serves as an excellent base for a suppressor host, given the threaded barrel. This configuration ships to Missouri and compliant US states following all applicable regulations.
Pros & Cons
Pros: The factory assembly guarantees reliability and fit. The component selection from CMMG, Magpul, and mil-spec parts creates a cohesive, duty-ready tool out of the box. The 12.5″ barrel offers a great balance of velocity and handling—you’re not giving up as much terminal performance as you would with an even shorter barrel.
Cons: The obvious one is the NFA paperwork, tax stamp, and wait time. KeyMod, while functional, has been largely superseded by M-LOK as an attachment standard, though plenty of accessory options remain. This is a complete upper, so you must pair it with a legal AR-15 lower receiver to build the SBR.
Frequently Asked Questions
Q: Is this a complete firearm?
A: No. This is a complete upper receiver assembly. To build a functional Short-Barreled Rifle (SBR), you must attach it to a registered AR-15 lower receiver. An FFL transfer is required for the lower receiver, and the assembled SBR requires an approved ATF Form 4.
Q: What’s the velocity difference with a 12.5″ barrel?
A: You can expect a loss of roughly 200-300 feet per second compared to a standard 16″ barrel, depending on ammunition. That said, common 55gr and 62gr loads remain supersonic and effective well beyond typical defensive engagement distances.
Q: Can I put a different handguard on it?
A: Yes, but it may require specific tools and know-how. The barrel nut is standard, so any free-float handguard designed for a mil-spec AR-15 upper should be compatible—just ensure it’s long enough to cover the gas block journal on this medium-profile barrel.
Ready to configure your next compact rifle platform? This CMMG upper is available at firearms-missouri.com for Missouri residents and nationwide.
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