Here it means a gun that has a very high value for some reason (rarity, engraving, commemerative, etc) it's kept in the safe and not fired. If you take a commemerative firearm and shoot it the value drops to maybe a little bit more than a comparable firearm that is not a commemerative. Original gas trap M1 rifles are pretty rare. If you had an old Colt SA that was made in the 1880s it would be worth say $2500 (enough to be a safe queen in my opinion). If you one you can prove was carried by Wyatt Earp in the same condition the price would be several hundred thousand dollars. Etc, etc, etc