Choosing the right caliber for your AR build is easier when you know the key differences between 8.6 vs 300 blackout and how a 5 inch 556 upper can make your rifle even more versatile. Both rounds offer unique advantages, and pairing either with a compact 5-inch 5.56 NATO assembly turns your lower receiver into a multi-mission platform.
When you compare 8.6 vs 300 blackout, the biggest difference is bullet size and power. The .300 AAC Blackout uses a .30-caliber bullet designed to fit standard AR-15 magazines without reducing capacity. The newer 8.6 Blackout steps up to an 8.6 mm (.338-caliber) projectile, letting you push heavier bullets at faster speeds. If barrier penetration or long-range impact matters most, the 8.6 vs 300 blackout debate often favors the 8.6 round’s extra energy on target.
Speed and noise levels are important too. From a 16-inch barrel, a typical supersonic 300 Blackout load reaches about 2,200 fps, while subsonic rounds drop under 1,100 fps for whisper-quiet shooting when suppressed. In an 8.6 vs 300 blackout showdown, 8.6 Blackout can send a 150-grain bullet at nearly 2,500 fps or drive a 300-grain slug at about 1,000 fps. That means greater impact at distance and solid barrier-busting performance. Both calibers tame recoil well in an AR platform, although 300 Blackout usually feels a bit softer thanks to lighter bullets and smaller powder charges. Add a quality suppressor, and both rounds become remarkably quiet for backyard ranges or tactical use.
Availability and cost affect your choice. Since 300 Blackout has been around longer, ammo, barrels, and accessories are widely available and generally more affordable. By contrast, the 8.6 vs 300 blackout newcomer can carry higher ammo costs and fewer factory barrel options—though makers are rapidly filling the gap with optimized 8.6-caliber barrels for both supersonic and subsonic performance.
Whatever caliber you pick, installing a 5 inch 556 upper adds serious flexibility to your AR. This ready-to-go upper features a 5-inch cold-hammer-forged barrel chambered in 5.56 NATO, a mid-length gas system for smooth cycling, and a free-float handguard with full-length M-LOK slots. It ships fully assembled, head-spaced, and test-fired—simply attach it to any Mil-Spec lower and you’re ready to shoot.
With the 5 inch 556 upper, you can swap uppers to change calibers or barrel lengths without buying extra rifles. Run a 5-inch 5.56 for tight-space drills one day, then swap in your preferred 8.6 vs 300 blackout barrel tomorrow—all on the same lower receiver. This modular setup saves money, bench time, and storage space.
The 5 inch 556 upper also supports lights, lasers, grips, and backup sights exactly where you need them. Its ambidextrous charging handle and dust-cover compatibility make it smooth to operate for both right- and left-handed shooters. When paired with match-grade ammunition, you’ll see sub-MOA groups thanks to its precision components.
In the end, choosing between 8.6 vs 300 blackout comes down to your mission. If you need maximum energy on target or barrier penetration, the 8.6 Blackout delivers. If you want a proven, affordable, low-recoil round with wide parts availability, stick with 300 Blackout. Either way, adding a 5 inch 556 upper creates a highly adaptable AR platform.
Ready to build your perfect AR? Visit Moriarti Armaments today to explore dedicated barrels and the 5 inch 556 upper, customize your setup, and place your order. Take control of your next shooting adventure with gear built for real performance.