I have an 870 Wingmaster 12 ga that I use for duck and goose hunting. It's **** near the perfect bird gun.
Here's what you do. You have to unscrew the cap that holds the barrell on. Underneath the cap you will should see a metal piece that looks almost like a set of crosshairs; this is pushed down into the chamber. A screwdriver may work if it's not stuck, however, a pair of needlenose pliers works quite well. Take the pliers, grab onto one of the edges and work it back and forth until it starts to come out. You will have to do this on all sides so that it comes out evenly. Watch yourself as this piece is spring loaded. You can poke your eye out.//content.invisioncic.com/y282845/emoticons/silly.gif.3549b64aaa5b6729eb8e865319a3234f.gif If you don't cover it with your hand the spring will send it flying. Once that is out take your plug and put it in the chamber with the flange side up. It should fit inside the spring. Then put the cover back on. You will need something to tap it lightly to get it all the way in. That's it, but for God's sake...make sure the gun isn't loaded! //content.invisioncic.com/y282845/emoticons/crazy.gif.c13912c32de98515d3142759a824dae7.gif
Good luck!