I would just like to add that a ram stack works by using the venturi effect to 'ram' more air into the carb by using the tapered part of the pipe and the rounded end is there to reduce the angel into the pipe as air doesn’t like sharp corners.
This about explains the effect of a stub stack, if you put sharp edges onto the airintake, the air doesn't flow neatly but creates an articifial restriction, by curving the intake all the way in you avoid this and make best use of the diameter/CSA available.