The other pistols being chambered in 410 are actually chambered in 45 colt, complete with rifled barrel, but with an extra-long chamber, so a 410 shell will fit. Since it has a rifled barrel, and is chambered in a pistol round, it is a pistol.
I don't know anything about the gun you have found, but if it's a smoothbore, it's a shotgun, and the "Short Shotgun" rules of the NFA should apply.
I suggest you get in touch with ATF for clarification.
Owning it would probably be neat. Don't know if it would be neat enough to counteract these two negatives, though. From www.atf.gov
, the firearms FAQs, the section on NFA.
(M7) May a private citizen who owns an NFA firearm which is not registered have the firearm registered? [Back]
No. The NFA permits only manufacturers, makers, and importers to register firearms. Mere possessors may not register firearms. An unregistered NFA firearm is a contraband firearm, and it is unlawful to possess the weapon. The possessor should contact the nearest ATF office to arrange for its disposition.
[26 U.S.C. 5861(d)]
(M8) What can happen to someone who has an NFA firearm which is not registered to him? [Back]
Violators may be fined not more than $250,000, and imprisoned not more than 10 years, or both.
In addition, any vessel, vehicle or aircraft used to transport, conceal or possess an unregistered NFA firearm is subject to seizure and forfeiture, as is the weapon itself.
[49 U.S.C. 781-788, 26 U.S.C. 5861 and 5872]