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.