I'll give you that, in a situation where you were engaging targets on the street where you may be poking holes in car doors or cutting through sheet rock and studs, the 7.62x39 is going to shine. But as far as room clearing, depending on shooter discipline, one could argue the "knockdown power" doesn't mean much. I think of it like this, in a room that is say 20'x20' or smaller, your maximum engagement range is less than seven yards so that 7.62 bullet is still screaming fast meaning it's more likely to just cut through soft targets' leaving a neat little hole and not transferring much of it's energy. The 5.45 with it's hollow cavity in the nose is likely to still yaw and tumble meaning it will transfer the majority of it's energy and not OVER PENETRATING which is something people overlook all too often. Another factor to consider is that in that situation, I wouldn't trust one round to do the job so I'm going give him a hammer pair or maybe even do a triple tap for insurance. So which weapon is going to allow me to get back on target the fastest? Probably that 5.45x39... that is how I justify using it in my mind anyway.
