Use a current weapon event like this
Code:
register_event("CurWeapon","fucntion_called","be","1=1","2=4");
It says that the weapon has to be a he grenade(4).
The first paramter is the event , the 2nd is the funtion that your hooking to, whatever fucntion youll be calling your code in , then the 3rd paramter is the weapon is active , then the conditions