Unless you’re talking under 12”, that 300 will have far less blast than a 20” Grendel, IME. maybe I missed where the 300 is a pistol? A 300 burns all its powder from most loads in under 10” or so, and case volume to bore, its very quiet at even supersonic, unsupressed.... at least compared to even 223, much less a Grendel.
i think hes more worried about a shorter barrel being more point-able in any direction by a kid, not muzzle blast. safety concern. counterpoint is that a shorter barrel is easier to handle, especially for a smaller person, i.e. a kid.
