Only if there is a state law that specifically requires that silencers be registered with the state or the federal government, then it is up to the state to prove whether or not the silencer is legal. So if someone in VA demands that I prove my silencers are registered, I can tell them to go pack sand.
Once again Ranb, most states that allow NFA weapons only do so IF they are registered under the NFA. Under the laws of most states, it is generally illegal to possess such weapons, with an affirmative defense to a charge of possession of an "offensive weapon" or unlawful possession of the specific type of NFA weapon carved out for those that can prove their possession is in compliance with the NFA. It is not the duty of the police or prosecuting entity to disprove all your possible defenses, nor is it to infer or assume your possession is legal just because you say so (without offering any proof such as a F4 or 5320.20). Now before you go assuming you know a state law and telling someone to pack sand, you need to look up the state law in question so you aren't the one getting packed ...into the back of their patrol car, handcuffed.
My state law (PA) defines NFA weapons as "offensive weapons". See PA Cons. Stat. § 908.
Prohibited offensive weapons.
(a) Offense defined.--A person commits a misdemeanor of the
first degree if, except as authorized by law, he makes repairs,
sells, or otherwise deals in, uses, or possesses any offensive
weapon.
(b) Exceptions.--
(1) It is a defense under this section for the defendant
to prove by a preponderance of evidence that he possessed or
dealt with the weapon solely as a curio or in a dramatic
performance, or that, with the exception of a bomb, grenade
or incendiary device, he complied with the National Firearms
Act (26 U.S.C. § 5801 et seq.), or that he possessed it
briefly in consequence of having found it or taken it from an
aggressor, or under circumstances similarly negativing any
intent or likelihood that the weapon would be used
unlawfully.
Now, since you specifically mention Virginia, lets look at VA Code Ann.§ 18.2-308.6.
Possession of unregistered firearm mufflers or silencers prohibited; penalty.
It shall be unlawful for any person to possess any firearm muffler or firearm silencer which is not registered to him in the National Firearms Registration and Transfer Record. A violation of this section shall be punishable as a Class 6 felony.
