I lump semi-auto and single shot together as they function pretty much the same way in game as you don't have to think about cocking the gun after each shot. They both have a set delay between shots. Honestly with a true semi-auto gun you can fire as rapidly as you can pull the trigger, but of course this is a game.
Let's see what you all think. Feel free to post up why as well, but try not to fight over which is best.