From the clear cote part of your post I am assuming that you have the high gloss finish on your rifle rather than the satin finish?

The black grip cap is just a plastic cap. There is really nothing that you can do about it to return it to original condition. The only thing I would suggest is to carefully cut around the edge of the cap (where it goes from the horizontal to vertical) with a very sharp razor blade and then carefully chip off the rest of the finish from the cap. Lightly sand to remove any blemishes.

Personally I would leave it alone.