I was wondering, if, while cleaning out on old house, barn or something, You came across a Thompson  Full auto machine gun of other such firearm. Could you legally register it and keep it if you pass all the class 3 stuff. It would have been  made before all the bans, when Americans had rights that are no longer valid. This has not happened to me,( BATF guys, please don’t smash my door in, THIS DID NOT HAPPEN, hypothetical situation) but I have always wondered about it.

I'd have never found it.

But seriously, the answer is NO. You can't register any more MGs.

the ATF actually adresses something about this on their web site.  The title is something like "I found a machine gun in grandpa's foot locker after he died"
Source at ATF's website: http://www.atf.gov/press/breakingnews/fy01/bn_aug0999b.htm

If it happens you start looking for the registration paperwork and if you are lucky you find it, in that case you can form 5 it to yourself tax free.

If you can't find any paperwork, you have an attorney contact the ATF with make model serial number etc... to find out if it on the Registery, if it is and registered to a dead relative you can form 5 it to yourself tax free.

those are just some of the things that can happen.

However if you find one regardless of what year it was made 1915 to 1986, if it was not already in the ATF registery, it must be disposed of.

If you do find something like this and get to the point you can't keep it, don't destroy the whole gun. Strip every part you can from the receiver with the serial number and destroy the serial number. The parts can be worth a great deal of money by themselves.