If you find a gun in a storage unit, there are certain procedures you must follow. First and foremost, clear it and make sure there are no unspent rounds in it. I would like reiterate that I'm not an attorney, nor do I have a FFL and that you should consult with a professional before making any decisions regarding firearms or antique weapons.
It is my understanding that if you find a firearm in a storage unit, you must turn it in to the local authorities or a Federally licensed firearm dealer. I recommend the latter because you have a better chance of getting it back. When you turn it in, make sure you get a receipt for it because if you're a non-felon adult, they will let you have it, after a waiting period and if it has not been reported stolen or used in a crime.
Although state and local laws may vary, any gun with a frame or receiver that was actually made before January 1st, 1899 is legally antique and not considered a firearm under Federal law. This refers to the actual date of manufacture of the receiver/frame, not just the model year or patent date marked. Any gun manufactured before January 1st, 1899, other than a machine gun or other NFA category, is not controlled in any way by Federal law. There is no Federal requirement for sales of these guns to be handled by Federally licensed dealers. Although State and local laws may vary, they may be bought and sold across state lines by private parties, regardless of what cartridge they are chambered in.
Telephone the Federal Firearms Licensing Center of the Bureau of Alcohol, Tobacco & Firearms at 1-866-662-2750 for specific questions. The FFLC can help ensure your guns are properly registered.