From what I see, with factory floorplates, A small amount of play between the magazine floorplate and the grip is necessary to push the magazine further in to move the retaining catch out of the way due to a small lip on the edge of the catch witch fits in to a groove in the floorplate. However, it seems on SOME R-9s, this "play" is excessive in that feed/jam problems and at least two (2) accidental mag jettisons have occured as noted on this forum. With the mod, absolutely no play is noted, nor is thier a machined groove for the "lip" of the catch to sit in.