300 Blackout vs. 5.56 NATO
When the trendy .30-cal earns its keep — and when 5.56 is still the smarter, cheaper default.
The Short Answer
300 Blackout was designed to do two things 5.56 can't do as well: run reliably suppressed from a short barrel with heavy subsonic loads, and deliver more energy from very short SBC/pistol barrels. If your goal is a quiet, compact, suppressed home-defense or truck gun, 300 BLK shines — it cycles subsonics that a short 5.56 struggles with and uses standard AR mags and bolts (you just swap the barrel).
For almost everything else, 5.56 is the better default: far cheaper ammo, flatter trajectory, more range, and wider availability. 300 BLK supersonic ammo is pricey and its effective range is limited. The honest split: build 300 BLK for a short, suppressed, close-range setup; stick with 5.56 for a do-everything carbine. And never mix the two in the same mags — a 300 BLK round can chamber in a 5.56 barrel and cause a catastrophic kaboom.