While this glitch is very inconsistent I have been able to do it numerous times.
All you need to do is grab a regular 12-gauge shotgun in multiplayer. Once you get one wait for another one to reappear in the same location or find a different one. Fire a few shots and reload while standing on the shotgun. If done right the shotgun will load past full capacity and even though it will say it is full, you can keep loading ammo you find into the shotgun. And you can load as many shots as you want! The screen will say you have no ammo available but all shots loaded in the gun will be able to be used. This glitch will take affect as long as you don't die or switch out the shotgun.
Please note that it may take multiple attempts to work.
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