Maybe we can add a lazer range finder for inf
Anyway the leaf sights max effective range is 200m with M16A1/2/3/4 and 250m with car15/M4/M4A1. This is because the flash hider is blocking your view when aiming at 250m. As for range>250m with the leaf sight, the scale simply doesn't go that far - 250m is max. The quardant sight can reach 400m though.
As for sharpshooting with a short barrel rifle, experience has shown that it's more effective than using the longer, heavier, bulkier M16A2. You're not expected to hit targets above 300m in real battle anyways, and the difference between M4A1 and M16A2 is insignificant in those ranges.
Actually, in Israel, every soldier that is expected to actually fire his weapon gets either a CAR-15 or an M4A1. M16A1s and scoped M16A2s are currently only used during basic training and courses, while galil types are only used by armor/artillery/etc, and even some of them use CAR-15s instead since they do some (simple) infantry tasks too (guarding in roadblocks and alike).
You should remember that sharpshooters aren't snipers - they're supposed to assault with the rest of the squad, which sometimes includes CQB, though it's also more comfortable to use carbine weapons even in open grounds.