Depends on where you are. In MOST of the US, it is legal to own fully automatic firearms IF you touch second base.
That means that the firearm must have been registered as a machine gun with the BATFE by May of 1986, you are 21, able to own a regular firearm, have approval of local chief LEO, transfer of gun to you is through a licensed machine gun dealer in your state, and you pay a one time $200 transfer tax.
Otherwise, penalties include 10 yrs in a Federal prison, fine is up to $250,000 now (I think) loss of right to ever own ANY gun, and you get a new roomate.
This also applies to converting a standard .22 semi-auto to full auto.