Unlimited Ammo for Heavy Weapons
Switch your weapon after you've fired and you will hit with the weapon you switched to and not the one you fired.
When you are going to attack, fire your weaker weapon (which has unlimited ammo) and switch to your heavy weapon (which as limited ammo). NO ammo will be used by the heavy weapon: this glitch lets it have Unlimited ammo as long as you switch to it from the weaker weapon every time you attack.