Start a Co-Op game on a level of your choice. Choose Marine-Grenade as your class. 
You have three classes of grenades: Normal, Frag, and Proximity Mine. Shoot off all 
rounds of your two least favorite types. After all of two types of grenades have been 
fired off, fire off all of your favorite type of grenade. After you are completely 
out of grenade ammo, press the button you have set as secondary fire. You should now
have a just about infinite amount of your favorite type of grenades.