Hi Macrow,
Mix a little polyurethane or spar varnish in with the tung oil. Then wipe it on with a rag, let sit for 10 minutes and then wipe off all the excess. Let dry overnight and repeat until you have a bit of gloss on the wood. It won't take long if you have some warm days. The result will be an attractive and authentic look. The other thing you can do that will make a big difference is add some pure yellow dye to the oil varnish mix. Behlens and others sell oil soluble dyes. That color will warm the cold purple brown of black walnut making it look much more like the French walnut these pistols were made from. This pistol is the same American black walnut but treated with yellow dye and finished with polymerized tung oil (not raw oil). It has the correct historical finish and color.
dave