I've finally figured out all the steps to correctly do the infinite ammo glitch.
First, get a rocket launcher(The ones I have used have the name "Hero" in them.) that has the description of "Consumes reduced ammo per shot."
Then, fill your inventory up until it's full.
Then, shoot it(Rocket launcher.) until you shoot it to the point where the NEXT shot won't deduct any ammo. For example, for the weapon I use, I can shoot it twice at 4, once 3, then twice on 2. So the ammo counter would be like this: 4/4/3/2/2/1/0 so I would stop at the FIRST 2.
Then, (haha lot of "then's") open up a chest, or gamble for a weapon in the Mad Moxxxi's Lounge, and hold X to swap it out for a weapon that you have NEVER equipped or used before or your choosing.
Voila, infinite ammo!
(Note: Haven't figured out how to do it with E-Tech weapons yet. :/ (Pinks.)