I know my cousin got one of the 3 1/2 mossberg slug guns and It took me a while to figure it out. eventually I took the magazine tube cap off and started loading the magazine the regular way. eventually the plug pushed out the magazine cap. it was a piece of wood, almost like a pencil with a rubber band on it. I'm not sure if that's the right way to do it, but it worked. maybe the 500's are the same.