Roundover bit I guess, if you can get one with a small enough radius.
I don't do it that way though, 'cause my rails taper and are not rounded as much at the tips.
Personally, I cut flash off with a utility razor knife, shape the rails with an orbital sander and 40 grit on a soft sanding block, then smooth out with 60, 80, 120 grit, then wipe down with epoxy thinned with isopropyl alcohol to give a hard shiny finish
Otherwise a roundover bit would be the call.
Either way use a strong dust collection system and/or a good mask.