Silencers are legal in all states, but 12 states limit possession and use to the military, police and licensed dealers. The other 38 states regulate to various degrees. For example Texas bans silencers, but registration is an affirmative defense to prosecution, so the BATFE allows them to be sold/made there.
A license (FFL/SOT) or a $200 tax is required to be paid to make or buy/transfer a silencer. For those of us who are not in the business of importing, manufacturing or dealing in firearms, the payment of the $200 tax on each one is the best way to go. The forms are avilable online and the BATFE approves any tax stamp application that is filled out correctly. A silencer can be tranferred tax free to your heirs or to a museum if it is left in your will or a trust is used.