Recently had the opportunity to try a .300 blackout rifle in an AR15 configuration. Here are my thoughts:
Recoil is a bit less than 7.62x39, but a bit more than 5.56.
Guy handloaded all his rounds. This lead to very spotty performance. Half the time, I was clearing jams.
Scope was mounted "low rise" on a flat top AR15. That was not good for cheek weld.
Scope was canted.
Other than that, it was a nice gun. If ammo were cheaper, I might consider it.
To be fair, there is not much it can do that a .223 cannot.