"300 blackout is joke of a cartridge"
I sure wouldn't go that far. Depending on the distance this round will do nicely for deer all while using 223 brass which is plentiful and cheap with the same magazines as any 5.56 AR. Swap tops with a 223/5.56 and you have 2 rifles in one.
I know you can form Grendel brass from 7.62x39 but I think the AR brass is cheaper and easier to get.
I have the 300BLK and the next gun is going to be a 6.5 Grendel, though.