I worked for a local PD in upstate NY. A widow came in with full auto Thompson in Cosmoline. It was a WWII bring back that her husband had. It was in the evidence room for a long time and the last that I heard. it was donated to the West Point Museum. It was an awesome gun.
If I found one, I would report it and already have a plan to have it donated directly to a big museum. Hopefully it would get chopped up. I would not just keep it. It is easy to say just keep it but you don't really know who would see it and then report it. What fun would it be to own the real deal Tommy gun and never show it off or shoot it? It is not worth going to jail for.