You do have to fit the interal of the stock inlet to the bottom metal. I used a dremel. A little more involved than I expected but it wasn’t too bad. It’s definitely not a straight drop in. But you also don’t have to change the overall inlet diemensionally externally on the stock if that makes sense. You remove material internally for the the part of the mag well that extends into the stock a little to fit. They have a video that’s does an ok job of showing what you do. And you do have to remove the ribs or the AICs mags themselves wont insert all the way. I’d think it’d probably still work with the original bottom metal but I didn’t try it