Varmint Hunter Magazine, Issue #75, July 2010, has a great article entitled "Silent Hunter."
You may be able to order back issues... Varmint Hunter Digital Edition | Digital Edition for Varmint Hunter Magazine | Online Varmint Hunter Magazine
The article addresses licensing, ownership, and legal use for each state which varies greatly.
a few noteworthy points made by the author...
Even when suppressed, there will be a sonic "crack" if the bullet exceeds the speed of sound.
Suppressors have been shown to add 30 to 50 fps due to added barrel length.
Recoil is often reduced by up to 50%.
David Whitson (Untitled Page
) makes them in nearby Plano, TX. He let my 14yo son shoot his suppressed 308 at the range. It was awesome!
IMHO - They are definitely worth looking into if you shoot a lot.