RE: What Barrel length?
If your 870 is like mine then the spring retainer can be removed with a pair of needle nose pliers and replaced simply by pushing it back in place. Be ready when you remove the retainer because the spring will be coming at you fast. I personally simply put a pencil in the hole to plug my magazine but it is not completely legal in TX since tools are not required to remove the plug. I have a 26 inch and love it.