I want to ask the obvious question.
If silencers are legal why would a civilian need one for a legitamate reason ?
The only people i can think of that can have a legal use for it can are police or military.
Fellas, fellas, can we stop it with the links to illegal pistol modifications? Nothing’s going to shut down this thread faster.
Why does somebody need a Lamborghini Diablo?
Why does Jay Leno need $45 million in cars and motorcycles?
Why does the owner of Dillon Precision need his own halftrack with a Quad .50 (four fifty-cal M-2 Browning machine guns) on an electric traverse-and-elevate anti-aircraft mount, mounted on the back?
Why does Don Trump need his own office building?
Why do people need a BMW Z3?
Why does Ted Turner need 150,000 acres in Wyoming?
Why does somebody need a 52" TV?
Why would a woman want a $65,000 diamond necklace?
Why would somebody have a 1.9-Ghz system and a 35 Gig hard drive just to surf the 'net?
Why would somebody spend a year and $50K on an off-road truck he drives once or twice a year, and even then only really slowly, over rocks and up river canyons?
Why would somebody have a $20,000 car stereo?
Why does a single guy need a 6,500-sq.ft house with three bathrooms?
Why would somebody vacation in the Bahamas?
Because you can, that’s why. 
It might surprise you to know that there are something like a hundred thousand fully-automatic firearms (“machine guns”) in private hands in the US right now. Why do people go through the paperwork, the background checks the hassle and the wait, just so they can pay $200 for the privledge of paying a further $5,000 to $15,000 for a “machine gun”?
Because they can.
I don’t know about California, but in most of the states that allow NFA weapons (inc. suppressors) they don’t ask any questions like that. In most states you aren’t required to need something in order to have it.
I used to work with a number of people in the chief counsel’s office of the ATF. They said if you were caught with a 2-liter soda bottle on the end of a gun, you were potentially in big trouble. I suppose unless you had the proper paperwork for your 2-liter bottle.
Also, you need to use subsonic ammo. Or you get little sonic boom. This will reduce your power a little bit. And you can’t use a silencer on a revolver. Too much gas (read sound) escapes between the cylinder and barrel.
I think that this is a legitimate question and I didn’t see any serious answers.
The sound that a firearm produces is really noise pollution. In general, we strive to make our various devices more quiet. Think mufflers on cars. Guns actually make a somewhat dangerous level of noise.
In my case I am taking shooting lessons. Both I and my instructor must wear ear muffs to protect our hearing. I can tell you for sure that this impedes instructor to student communication.
I’ve been told that hearing protection is especially required indoors. My eventual use for my gun will be self-defense. I would hate to deafen myself because I couldn’t get my hearing protectors on in time.
A quieter gun would be a better gun. And there’s no need to go all the way to silent.
What a damn mess you people have made of this General Question.
BingoBurringo We do not allow posts that instruct people how to circumvent the law. The products you linked to are quite obviously aimed at people wishing to avoid the BATF regulations. The products being sold may not be illegal, but we do not allow such links here.
Tony Montana, you are required to cite your source when quoting. The more coherent part of your posts appears to come directly from http://www.advanced-armament.com/owninfo.html
kalashnikov, I honestly don’t know about the legality of your links, but I’m deleting them too, just on general principles.
Doc Nickel, first, this is not Great Debates, and second, encouraging illegal activity is forbidden there too.
I hope the OP’s general question has been answered, because this thread is closed.
bibliophage,
moderator, GQ