An adapter to mate two patented products together shouldn't infringe any patents. It would be the same legal grounds as creating an aftermarket magazine or furniture set to fit a patented firearm. Unless the adapter copies a unique and patented attachment method or specific unique design that's patented directly it should not be an issue. The locking method on the mossberg magazines are likely not unique or patented. If they are the patent numbers should be on them to look up to see what design features are specifically covered by the patent.