You have 3 points . "Low" "middel" "Fast" .
(Its not a big difference between Low and Fast .. only other way's .. Try it out

)
Select Low and turn Unl.Ammo on if you want ..
NOTE : UNLAMMO CRASHES AFTER ROUND AND DON'T WORK IN ZOMBIEMOD !
Now Take Nades ( Grenades ) out and Shot them .
you will see you shot mutch faster then normal and the game will not crash .
(maybe you shot 5 - 10 seconds and then a litle break [better if you never want a crash 100%])