Description
The SIG M400 AR-15 in .300 Blackout with a 16-inch barrel and 30-round magazine is a purpose-built, factory-ready rifle designed for versatility. It bridges the gap between a standard 5.56mm AR and a dedicated close-quarters platform, offering superior terminal ballistics with subsonic ammunition while maintaining a familiar, reliable AR-15 manual of arms. I’ve put several hundred rounds of both supers and subs through this configuration, and it’s a solid, no-nonsense foundation for a serious do-it-all rifle.
Key Features
- Chambered in the versatile .300 AAC Blackout caliber.
- 16-inch cold hammer-forged barrel for durability and accuracy.
- Includes a 30-round SIG SAUER polymer magazine.
- Built on SIG’s proven M400 platform with a mil-spec lower receiver.
- Free-float M-LOK handguard for accessory mounting.
- Two-stage match-grade trigger for a clean, predictable break.
Specs & Details
| Caliber | .300 AAC Blackout |
|---|---|
| Barrel Length | 16 inches |
| Overall Length | 32.25 – 35.5 inches (stock extended/collapsed) |
| Weight (unloaded) | Approx. 6.7 lbs |
| Magazine Capacity | 30 Rounds (included) |
| Trigger | SIG Two-Stage Match |
Here’s the thing—the .300 Blackout round was designed for a specific role, and this SIG M400 executes it well. The 16-inch barrel is the sweet spot for this cartridge, allowing you to maximize velocity with supersonic loads for hunting or defense, while still being an excellent host for a suppressor with heavy subsonics. The rifle is mil-spec compatible, meaning you can swap out stocks, pistol grips, and triggers with a massive aftermarket of AR-15 parts. The included handguard is a solid piece of kit, giving you plenty of real estate for lights, grips, or a laser. At a price of $1139.55, you’re getting a complete, ready-to-run system from a top-tier manufacturer, not a bare-bones kit.
Who This Rifle Is For
This is for the shooter who wants one rifle to handle multiple jobs. It’s ideal for the Missouri hunter pursuing deer or hog at moderate ranges, where the .300 Blackout’s heavier bullet outperforms the 5.56. It’s also a fantastic choice for the home defender who plans to use a suppressor—subsonic .300 BLK through a can is famously quiet. Finally, it’s for the AR enthusiast who appreciates a quality factory build as a starting point for customization without needing to build from scratch.
Pros & Cons
Pros: Outstanding factory trigger. SIG’s barrel quality is top-notch. The .300 Blackout caliber is incredibly flexible. The rifle is drop-in ready—just add optics and ammo. The M-LOK handguard is modern and lightweight. It’s a proven, reliable platform right out of the box.
Cons: .300 Blackout ammo is more expensive and less common than 5.56mm. If you don’t plan to suppress or hunt, a 5.56 rifle might be more economical. The rifle ships with iron sights, so you’ll need to budget for an optic.
Frequently Asked Questions
Q: Will this rifle cycle both supersonic and subsonic .300 Blackout ammunition?
A: Yes, it’s designed to. The gas system is tuned from the factory to handle the wide pressure range between hot supersonic loads and low-pressure subsonics, especially with a suppressor attached. I’ve had zero issues switching between the two.
Q: Is the upper receiver compatible with other AR-15 lowers?
A: Absolutely. The SIG M400 upper uses a standard mil-spec AR-15 pattern. It will mate perfectly with any mil-spec lower receiver, like those from Aero Precision, Palmetto State Armory, or Colt. This gives you huge flexibility for future builds or upgrades.
Q: Is this rifle suitable for a first-time AR owner?
A: It can be, if you’re committed to the .300 Blackout caliber. The manual of arms is identical to a 5.56 AR-15, so all the controls and handling are the same. Just be prepared for the higher cost of practice ammo compared to a 5.56 rifle. The build quality and included features offer great value.
If you’re looking for a capable, modern AR-15 that steps beyond the ordinary 5.56 platform, this SIG M400 in .300 Blackout is a compelling choice. It’s available at firearms-missouri.com for Missouri residents and nationwide. Check the product page for current availability and to start the simple FFL transfer process.
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