It's easier to alter trajectory on a short barrel weapon than it is with a long barrel weapon. Simple geometry, really.
Actually its easier with a longer barrel. The longer barrel increases the sight radius as well as allowing the propellant gas more time to expand within the barrel this causes the bullet to have a higher final velocity. The increased velocity flattens the trajectory. A flattened trajectory effectively makes it easier to aim and adjust. Hence why you don't see sniper rifles with short barrels.
Aren't we talking about dicks?
If we are, why are there propellant ga-
You know what, never mind. Don't even wanna know.