5.56 NATO vs 300 Blackout: which AR-15 caliber should you choose?

I build AR-15s for a living, and the 5.56 versus 300 Blackout question comes up at my counter about once a week. I’m Lance Rankin, owner and gunsmith at Western Sport in Roanoke, Texas, and I own and shoot both calibers. Here’s the honest version of the comparison, without the marketing gloss.

The short answer

If you’re buying or building one general-purpose rifle, get 5.56. If you’re building something short, something suppressed, or a dedicated hog and deer gun for Texas distances, 300 Blackout starts making a strong case. And because this is an AR, you don’t actually have to choose forever. More on that below.

The case for 5.56 NATO

The AR-15 was designed around this cartridge, and everything about the platform reflects it. Ammo is everywhere, from match loads to bulk range fodder, and it’s the cheapest centerfire rifle practice you’ll find. Brass-case plinking ammo runs somewhere around 35 cents a round as I write this, less for steel case. The load I stock deepest for that job is Federal XM193 55-grain M193 ball, the same 5.56 spec the military trains on. That matters more than people admit, because the shooter who practices monthly with a cheap caliber beats the shooter who practices twice a year with an expensive one.

Recoil is mild, which makes it easy to shoot fast and easy to teach on. It’s genuinely effective out to around 500 yards from a 16-inch barrel. Two caveats from the bench. First, 5.56 depends on velocity for its terminal performance, so very short barrels cost you real effectiveness. Second, cheap FMJ is range ammo, not defensive ammo. If the rifle guards your home, load it with a quality expanding or fragmenting load.

If this is your first rifle, start with my best AR-15 for beginners guide. The answer there is a 5.56 gun, and it’s not close.

The case for 300 Blackout

300 Blackout was developed by Advanced Armament Company to solve two specific problems: getting real power out of short barrels, and playing nicely with suppressors. It does both better than 5.56 ever will.

The cartridge burns its powder quickly, so it gives up very little from a 9-inch barrel. That’s why it dominates the AR pistol and SBR world; my AR pistol roundup is full of Blackout guns for exactly this reason. And it runs bullets from 110 grains up to 220-grain subsonics. Pair those heavy subsonic loads with a suppressor and you get a rifle that’s genuinely quiet, not just hearing-safe on paper. Supersonic and subsonic feed from the same rifle and usually the same magazine, no parts swap required. As a Type 07 FFL and SOT, suppressors are a big part of our business, and 300 BLK is the caliber I recommend most often to first-time can buyers.

The downsides are cost and reach. Blackout runs roughly two to three times the price of 5.56 per round, and it’s a 300-yard cartridge in practical terms. It’s not a volume-training caliber unless you reload.

The safety warning I give every buyer

Read this part twice. A 300 Blackout round will chamber in a 5.56 barrel, and firing it that way can destroy the rifle and injure you. Both cartridges use the same bolt and fit the same magazines, which is convenient right up until the moment it isn’t. If you own both calibers, keep the ammo rigidly separated: different magazines, marked and dedicated, ideally different colors or brands, and never a loose round floating in a range bag. I’ve seen the aftermath of a Blackout kaboom on my bench. The gun was scrap and the shooter was lucky.

Home defense considerations

Counterintuitively, 5.56 with proper defensive loads is often the safer indoor choice, because lightweight fast bullets tend to fragment and dump energy quickly, reducing over-penetration through walls compared with heavier projectiles. Subsonic 300 BLK through a suppressor, on the other hand, protects your hearing in a hallway better than anything else on this list. Both are defensible choices with the right ammo. My home defense AR setup guide walks through the whole configuration, not just the caliber.

You can have both

Here’s the good news the caliber-war crowd skips: the two cartridges share bolts, magazines, and lowers. A complete 300 Blackout upper drops onto your existing 5.56 lower in seconds with two pins. Build or buy one quality lower, then run a 16-inch 5.56 upper for training and general use and a short suppressed Blackout upper for the fun stuff. My AR-15 build guide covers the process, and building uppers to order is a big part of what my shop does.

Bottom line

5.56 for your first rifle, for training volume, and for all-around use. 300 Blackout for short barrels, suppressors, and hog control. Both if the budget allows, with your ammo storage sorted out like your safety depends on it, because it does. Want help speccing a build or picking a suppressor to go with it? Call me at 817-393-4000 or email [email protected], and check the online store for current inventory.