I have a Stevens 94F 12ga single shot shotgun. Says on the barrel it takes a 3" shell. Are all 3 " shotgun shells considered a 'magnum"? I was given 4 boxes of new 3" 12ga magnum shells and was hoping that my Stevens can use them. I don't want to mess anything up. The gun is in very good condition. My step father gave it to me many years ago. I've only been using 2 3/4" shells so far.