Using entity as a gun bullet?

So I'm trying to mke some guns for my gun mod, I could make simple ranged item weapons, but then i wouldnt be able to make reloading custom firing speed and shotguns possible, so I came up with an idea, I would make a mob that when it spawns it would tarvel forward, the direction you are looking dealing damage to mobs it hits(right-clicking with the gun would spawn the mob). But, I don't seem to find any way to do this, I know how to make it travel for example South or North but i want to make it traveů the same direction as teh player is looking. Please help.