I read about a .300BLK in a 5.56 once, and owning both I am aware to not be "that guy". I'd like to think that if you did not notice that one short fat .300BLK round while you are loading the mag...your situational awareness is probably sub-par. Just sayin'...I know a guy that used one once. Assisted a 300bo into a 556 chamber. Pulled the trigger & things got ugly. Luck was with him & just messed up his hand,the rifle wasn't as lucky.
It was a new box of 556 ammo that had a 300 bo round in it & he didn't notice it when loading the mag.