Guys, any fixed silencer on a firearm is highly illegal unless you go through the paper drill and pay the fees to have the ATF bless one particular gun, usually a rifle, for silencing. I recently ordered a lever action in 45 Colt for some hunters who got a contract to thin down Coyotes on a nearby Air Base. The 45 Colt silences easily because it is a sub sonic round and also has a relatively short range since they didn't want to take any chances with long range overshoots or ricochets. Notice the term 'sub sonic.' It is much easier to silence such a round which translates to 22 short, most 9mm ammo, 38 spl, and 45. The hotter rounds, handgun or rifle, which break the sound barrier add that miniature sonic boom or a 'crack!' in this case, to the sound suppression problem. Military silencers on rifles like a 308 are big mothers with up to an 18 inch or more long, quite large 'muffler' attached to try and quiet the round. Some rifles like that have a sleeve along the entire barrel with a series of holes to leak gas along the way and have the bullet sub sonic before it leaves the muzzle. It is a problem with supersonic rounds.
For a 22 short, a bunch of kid's balloons from the dime store do a fair job but you need one or two taped tight over the muzzle for each shot. As far as I know, there's no law against this sort of one time 'silencer' but I could be wrong.
It is legal to silence air rifles and I have a Gamo break barrel air rifle with integral silencer of the muffler type. It cuts the sound about 50% or so but you can still hear a fair bang, mostly the sound of the big spring doing its thing.
My recommendations for silent hunting are, first choice, a really good blowgun; second choice, a small cross bow. The Cold Steel .625 inch (or caliber) blow guns are excellent and the gun, with its crutch tip on the end, makes a good walking staff or even a fighting staff in a pinch. I have completely impaled rabbits and prairie dogs with mine using both the steel and bamboo darts provided with it. It's also deadly accurate out to 20 plus yards. I shoot mine at that range routinely and I have to space my darts or I'll ruin them by hitting one dart with another and thus ruin the plastic funnel device on the rear. In a sure nuff survival situation, my darts or bolts would be tipped with one of several fast acting poisons real quick since I don't like chasing wounded prey.
It's also easy to make a good blowgun with a five or six foot long length of electrical conduit of about one-half inch ID. I make my darts from 8 penny nails, plastic from notebook inserts, and Scotch tape. These are wicked too.