Hi,
This like other finishes labeled "interior" is because it does not contain UV blockers. True-Oil, Chamber's oil, boiled linseed all, boiled linseeed oil varnish, polymerized tung oil, all original finishes etc, etc, etc, etc, do not contain UV blockers and would be labeled for "interior" use only. If you intend to use your gun as part of your deck railing, I suppose you might want those UV blockers.
dave