They are not exactly rivets, as I understand they are pins that are pressed in place. I believe that a special tool made of hardened steel is inserted in the magazine well, then the pins are pressed from each side by a vice. This make the pin expand and stay put. I once saw one of these tools on ebay, but didn't know what it was at the time, now I wish I would of bid on it. It's not a very good design and I am not surprised if one of yours is loose, but not to worry as it is held in place by other things. If you don't have a tool and new pins, don't take it out!