Just my opinion but a carbine that is easy to handle at powder burn range is going to be a compromise at 600 meters. Likewise, a solid 600 meter performer is likely to be longer, heavier, and harder to handle for extreme close range.
I'm confident in my 16" carbines to 300 meters but that confidence wains beyond 400.
Last summer, we were shooting at some clumps of grass in flood water out to an estimated 600 yards. Not many hits but close enough to cause concern (within 3' of intended target). I'd say that we'd have seen more hits if using a more solid support but this was real life shooting off crossed sticks. In comparison, shooting same carbine, ammo, and shooting sticks last week, I was hitting 100% at 300 meters on 8" swingers.