Serial number was taken off by the clerk who the weapon was surrendered to. Simple as that.
if any of them are automatic, and we are going to assume they arent, but if they are the BATF will de-mill them and destroy them if they ever find out about them. Beyond that Noefl is right, I am pretty sure I know, but not going to give out legal advice that could be wrong and put you in jail. Just remember, the any NFA guns have to be reported to the ATF so they can de-mill/destroy/ruin them.