Normally, with the Infinite Ammo cheat turned on, your weapons do not gain EXP. However, if you play with the Infinite Ammo cheat turned off and keep using your weapon until it has no ammo left, you can do a trick to get back up to full ammo for free. Simply turn the Infinite Ammo cheat on once your weapon has zero ammo, and fire the weapon a number of times equal to that weapon's maximum ammo count. So, if using the Fusion Grenade, fire it seventeen times with the Infinite Ammo cheat turned on, and then turn it back off. Your weapon will be back up to full ammo, and you can then use it to gain more EXP. This is very helpful in Challenge mode.