The record for the longest confirmed sniper kill was made with a MacMillan Tac-50 rifle, if that's any indication of what the best long distance rifle would be. Of course, given the broad nature of the term "gun", and that it's often applied to field artillery pieces, as well, one could just as easily say the M1 240mm howitzer would be the best.
yes
yes. the gun will shoot but the bullet will not go to far due to the friction.
some shoot far and some dont so they all dont shoot the same length
Miles
a couple of hundred meters
About 15+ feet.
A shotgun can shoot up to 80 yards it it depends on what the choke is on the gun.
They can shoot as far away as they want. There have been successful shots at well over a mile.
About 100yd
up to a half mile
Impossible to answer without knowing the caliber.
Accurately, about 100 meters.