CT looks good to go for silencers, I know they allow machineguns if they are full auto only, easily done in a M16.
Where I am a silencer can be permanantly welded to a short barrel and it becomes not a short barrel if the barrel/silencer overall is greater than 16". It is also a flash hider by regs federal, so you will need check with CT to know if this can be put on a semi auto.
I would contact this man, he will know all.
http://www.ctgunlawyer.com/nfa_trusts.html