I'm not really sure.
Few people say DC is RS 2.0, and I think RS manages somewhat okay without a B-slot, right?
All I know is you definitely need Rumble Shot, Comet Crasher and Bullet Blitz at all times. The 4th skill could be Leg Shot or Reload. With that it should be fine.
With a B-Slot extra skills would be Leg Shot/Reload depending what you took first, Dual Buster (noted) Magic Adrenaline, Head Shot, and a 300 MP skill of choice or Artillery Strike. These 4 are not completely necessary though they add to combo variety or extra damage depending on the situation, dungeon or PvP. So I'm guessing DC can handle things a little easier than a RS would.