Blackout is a relatively new cartridge with a unique history. The caliber is a result of the military’s experience using the 5.56mm round in M16. .300 Blackout was specifically designed to work well with the M4/AR15 style of guns. The round is built on the tried and true .223 cartridge but with a wider neck that allows it to accept .30 caliber bullets. As a result, .300 Blackout ballistics are very similar to the 7.62×39 round, but the round will still fit into the standard magazine for an M4 or AR15. imr 4831
However, the.300 Blackout round (or 300BLK for short) can serve two roles. 300BLK has supersonic bullet speeds when loaded with lighter bullets in the 120 to the 130-grain range. This means that while the sound of the round firing is quieter when using a suppressor, there is still a very loud “crack” when the bullet breaks the sound barrier. 300 Blackout is designed to use heavier bullets in the 190-230 grain range which travel at subsonic velocities, yet still, work well in the M4/AR platform.
That’s quite the technological achievement. The vast majority of AR-style guns use either a direct impingement or gas piston system to cycle the action and chamber a new round. These systems are designed to work with a specific range of energy. That energy is delivered to them from the gases siphoned off by each round firing. Too much gas siphoned off (and too much energy delivered to the action results in feeding issues and low muzzle velocity. Too little gas siphoned off, and the AR doesn’t have enough power to load a new cartridge. What the creators of the 300 Blackout managed to do is create around that delivers enough energy to work reliably in an AR with both high-speed, supersonic rounds and heavier, slower-speed subsonic rounds.