You might have to dremel a bit that that dimple is. IIRC, that's a small spot weld. I could be wrong. One of the best ways to get that cap off is to let the whole thing soak for a good long while in some kind of lubricant/penetrating oil, then fix the plate in a vise and use a flat pry-bar on the cap. When I did mine, I lag-screwed the buffer assembly to a board, and locked the board in a vise. A medium-sized flat bar is just thick enough to fit in those two gaps in the cap, and long enough to give you leverage to turn it. Even after Dremelling the spot weld, you still have some weld to break loose. It'll go. Just make sure that whatever tool you use to turn it fits those two gaps properly, so you don't bung them up.
I saw a post on cetmerifles dot com where a guy had made a special tool to turn that from of a piece of key-way steel welded to an old 3/8" socket.