.30 caliber round derived from the 5.56 (.223 caliber) NATO round.
https://en.wikipedia.org/wiki/.300_AAC_Blackout
In a nutshell, they shortened a 5.56 NATO shell casing, and increased the neck size to accommodate a .30 caliber bullet.
The .300 BLK isn’t legal for deer in Michigan’s southern zone, but .450 Bushmaster is. Go figger.