In this case the higher the number the faster it kills them. A good fire rate is 9.9 for guns that shoot bullets, but for guns that shoot rockets making them fast is pointless cause you waste so much ammo. In my weapon file I put all the slow rate of fire guns to like 2.2 so it fires a little faster.
I heard of people setting it to 100+ and they would empty their clip of 16000 ammo in 1 second.
When I made ranked mods for a friend. I set them to this
<rates firerate="9999" blindfirerate="9999"/>
and change the aiming accuracy to 0.0 for perfect aiming at first but if you hold it down then it will get inaccurate like a regular one. You can kill someone within half a second. Just tapping the trigger kills someone right away. Accuracy + long range + firerate = nice advantage .