DO NOT USE THIS WAY. LOOK DOWN FOR MUCH EASIER WAY. THIS IS FOR ARCHIVES PURPOSES ONLY Okay, first you have to have a large machine gun with a bi pod on it. Go prone and it'll give you the option to set up the bi pod. Move foreword while holding x to set it up. Make sure you hold x until it is completely set up. Then the ammo on your screen should disappear. This glitch is known about by Treyarch but won't be fixed until the next major update. This does work, as I was just doing it now, and it's amazing.
Edit: Heres an easier way.
THIS ONLY WORKS MOUNTED ON THE GROUND BTW.
Get on the ground. Get Mounted up. Push the left analog stick foreword. Your ammo will disappear. Enjoy.
The weird part about that is I recently watched a video where a guy was playing zombies and was using that glitch in the game. You can see that his ammo was not going down and he had the gun mounted. Not sure which platform he was using though. That would be the ultimate weapon in zombie mode for sure.