You cannot do it, PERIOD.
It must be done by a Class3 weapons dealer, after the conversion is done, you may apply with the BATF through your dealer for a Class3 weapons permit, the cost is $200
After that, the weapon may not be transferred to anyone except your heirs without the same deal again, a $200 conversion fee.
The BATF calls this a "Tax Stamp"
The price is the same for;
automatic weapons
silencers
hand grenades
rocket launchers
bazookas
tanks
jet fighters
nuclear submarines
j/k on the last 3