I really wish all weapons fired "full-auto" when you hold down the button, much as how tools were changed to work.
That said, there really should be no difference between holding down the button and rapidly clicking, when it comes to tools. And when I rapidly click with a weapon, occasionally I will get a "ghost fire" where it doesn't fire, but causes the weapon cooldown.
If there is a difference, it needs to be fixed in the game.