I'm inclined to say leave it alone.
If you do decide to carve it, I think you should plan to refinish the whole gun. Strip completely, carve, re-scrape, burnish, then stain and seal. Then finish with varnish,oil, Permalyn, whatever.
You may, if you are very careful, be able to incise carve right through the finish, and then just apply finish to the raw cuts. But one slip, and you've got a scratch on your finished wood. I dunno, I never tried this.