I would have no problem taking a shot at 400 yards. My 300 win mag is rated for 1,000 yards, and 400 is just like a walk around the block.

Seriously, I have shot my AR from 300 and 400 yards with pretty reliable accuracy. I agree with several other posters that 400 yards isn't as far as it sounds. Now, I wouldn't take a 1,000 yard shot. It really boils down to the shooter.