Description
You’re clearing a property line, moving through dense timber where shots need to count and over-penetration is a real concern. The standard 5.56 round can be a liability here—too much speed, not enough punch at subsonic velocities. You need a tool that bridges the gap between a carbine’s handling and a heavier caliber’s authority. That’s where the purpose-built .300 AAC Blackout cartridge shines, and the CMMG MK4 T SBN (Salt Bath Nitride) .300 AAC Blackout 16″ Medium Weight Taper Profile Threaded Barrel Black Furniture 30rd is the rifle built to harness it. This isn’t a parts-bin special; it’s a dedicated, mid-length gas system AR-platform rifle engineered from the ground up for the .300 BLK’s unique ballistic profile, offering formidable short-to-medium range performance in a familiar, reliable package.
Here’s the thing—the .300 Blackout was designed for versatility. With supersonic loads, it delivers energy comparable to the 7.62x39mm. Switch to heavy, subsonic rounds paired with a suppressor, and you have a phenomenally quiet system that still hits with the authority of a .45 ACP. The CMMG MK4 T is the vehicle for that flexibility. Its 16-inch medium-weight tapered barrel provides the optimal dwell time for reliable cycling across the entire spectrum of .300 BLK ammunition, from light varmint rounds to 220-grain subs. The Salt Bath Nitride (SBN) finish isn’t just for looks; it creates an exceptionally hard, corrosion-resistant surface inside and out, significantly extending barrel life and simplifying maintenance. At $833.49, this rifle represents a serious value for a dedicated .300 BLK platform with these features.
Built for the Task
The heart of this system is the 16-inch barrel with its mid-length gas system. This configuration is ideal for .300 Blackout, providing a smooth, reliable impulse that’s less harsh on components than a carbine-length system, especially when running suppressed. The threaded 5/8×24 muzzle is ready for a muzzle device or suppressor mount. The black furniture includes CMMG’s rip-stock buttstock and pistol grip, offering a solid cheek weld and control. Crucially, this upper is mil-spec compatible, designed to pair seamlessly with any standard AR-15 lower receiver you already own—think Aero Precision, Palmetto State Armory, or Colt—with no gunsmithing required. It fits out of the box, making your build or upgrade straightforward.
| Caliber | .300 AAC Blackout |
|---|---|
| Barrel Length | 16 Inches |
| Barrel Finish | Salt Bath Nitride (SBN) |
| Gas System | Mid-Length |
| Muzzle Thread | 5/8″x24 |
| Furniture | Black Rip-Stock & Pistol Grip |
| Magazine | 30-Round PMAG (Included) |
Pros & Cons
The Good: Dedicated .300 BLK platform eliminates feeding/cycling guesswork. Salt Bath Nitride barrel treatment offers superior durability and ease of cleaning. Mid-length gas system ensures reliable operation with both super and subsonic ammunition. Complete with magazine and ready to mate with your lower. Excellent foundation for a suppressed hunting or defense rifle.
The Considerations: .300 BLK ammunition is more expensive than 5.56x45mm. The cartridge’s full potential is realized with a suppressor, an additional investment and paperwork. As a complete upper, it requires a separate lower receiver to function.
Who This Rifle Is For
This CMMG is for the shooter who understands ballistic application. It’s for the Missouri hunter pursuing whitetail in thick cover where shots are inside 150 yards and a heavier, slower projectile is an advantage. It’s for the home defender who values terminal performance and the future option of whisper-quiet suppression. It’s for the AR enthusiast who wants a dedicated, no-compromise upper for a specific role, rather than adapting a 5.56 rifle. This package ships to Missouri and compliant US states, ready to become your go-to for medium-game hunting or a formidable defensive tool.
Frequently Asked Questions
Q: Will this upper work on my existing AR-15 lower?
A: Yes. The CMMG MK4 T SBN is built to standard mil-spec dimensions. It will pair perfectly with any mil-spec AR-15 lower receiver. You simply push the takedown pins, swap uppers, and you’re ready to go. Remember, you must use .300 AAC Blackout specific magazines or marked mags for reliable feeding.
Q: Is the barrel threaded for a suppressor?
A: Absolutely. The barrel features a standard 5/8″x24 thread pattern, which is the common thread for .30 caliber muzzle devices and suppressors. It comes with a protective thread cap installed.
Q: Can I shoot supersonic and subsonic .300 BLK ammunition through it?
A: The mid-length gas system on this 16-inch barrel is engineered to handle the wide pressure variance between supersonic and subsonic rounds reliably. You may need to adjust your buffer weight when moving between extreme load types, especially when adding a suppressor, but the system is designed for this versatility.
Ready to build your purpose-driven .300 Blackout rifle? The CMMG MK4 T SBN upper is in stock and available at firearms-missouri.com for Missouri residents and nationwide. An FFL transfer is required for the lower receiver to complete your build.
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