The rifle won't matter that much. The Squad Automatic Weapon that'll have to go with it will.
I wonder if we're going to see a move to a .30 caliber cartridge based on the 45mm long 5,56 M193/M885 case. The .30 Blackout would seem to be the likely contender.
1) My personal experience with .300BLK = 0
2) Buddy just built a .300BLK rifle ... maybe I’ll get to play with it.
3) Ballistics table says it’s a hair weaker than 7.62x39 ... 125gr @ 2200fps vs 124gr @ 2350fps.
If you’re looking for a ‘reach out and touch someone’ round, I don’t see .300BLK as answering the mail.