How to make mobs move in a straight line, and be affected by gravity?

Hi,

I am trying to make a ranged weapons mod. However, since I don't really like the ranged weapon elements, I am trying to make my own system.

I have made and retextured a slime, which will act as my ammunition for now, but I am unsure how, when a person spawns it by using an item, it moves forward in a straight line, while obeying gravity.

Was wondering if anyone had any ideas as to how I could do this?

Thanks.