There was an outfit back in the '80s that made tube magazine plugs for shotguns. They were tubes of aluminium of the proper length for whichever model of shotgun it was for. In addition, the tube was threaded at both ends for the appropriate choke tubes for that model of gun so that in addition to limiting you to 3 shots you also always had 2 spare choke tubes along if you wanted to change choke. I thought that it was a great idea but I only ever saw 2 of the tubes.
