except you're getting 16" 7.62x39 ballistics out of a 10" barrel. If you want rifle speed out of a short barrel, very few things do it as well as 300.Ballistically, 300BK unsuppressed is like shooting an AK47 unsuppressed. Similar bullet weight, similar powders and charge. 10.5 barrel is no problem - you can expect that a standard commercial load will have burned almost all of the powder before the bullet makes it to the muzzle.
Plinking 300 is about 50 cents a round while 7.62 is about 43 cents so you are paying a premium for 300 at the moment.
subsonics are hitting about like 45acp but can punch through soft armor. Really fun, but not the best use case for 300 IMO