I once did quite a lot of jewelling (engine turning) on rifle bolts, shotgun bolts, pistol barrels, revolver hammers, and the like. Jewelling was originally used on the inside of shotgun locks and pistol/revolver actions to hold oil for lubrication. Then it got to be used outside, mainly for show. The fad seems to have gone away, partly due to the fact that the jewelling on a moving part, like a rifle bolt, will almost inevitably be scored or marred by the movement of the part in the action, and the result will look (IMHO) like heck. Jewelling looks great if the gun is to be hung on the wall and admired; for a practical gun, I usually advise against it.
Jim