Basically the macro is like you "tapping" but makes it so you just have to hold your fire button and without delay of you lofting your finger... If that makes sense.. The (ms) determines the delay between each shot. So 1 (ms) has a 1 millisecond delay between each shot of your gun. 100 (ms) being 100 millisecond delay between each bullet fired. As for higher or less it all depends on the gun you use. For a para I would be going for a medium set number. As for a smg I'd keep the number low. If the number is really low you might be accused of silent shotting with an smg because you will be shooting more bullets than the game record the bullet loss.