My recommendation would be gold gilt. It can be an expensive process due to the cost of the gold leafing, but the amount of gold leafing you would need is insignificant.
The basic process is to clean the surface, apply a thin surface of glue (only inside engraved surface or lettering), rub in the gold leaf (apply gold leaf to gun engraving and wax paper on other side, where your burninshing tool or rubbing finger is at, and rub it in), and finally apply a sealer to keep it protected.
Below is a picture of an antique French clock I restored using the process. Notice wth hunter with his gun and the rabbit. The most difficult part of my project was preparing the surface for gilting. Doing the roll marks or engraving on a gun should be a snap. Depending upon the amount of detail, you might even want to apply a wax to the non-engraved suface to keep the gold inside the lines so to speak. It is easy to correct a mistake, a slip, or even remove if you do not like the look.
Last edited by lamarw; June 27, 2012 at 09:32 AM.