AL, AR, AK, AZ, CO, CT, FL, GA, ID, IN, KS, KY, LA, ME, MD, MO (with C&R), MS, MT, NE, NV, NH, NM, NC, ND, OH, OK, OR, PA, SC, SD, TN, TX, UT, VA, WA (bans use), WV, WI and WY allow unlicensed persons to own silencers.
Check out
http://www.silencertalk.com and
http://www.subguns.com for forums, legal summaries and other stuff. I make silencers as a hobby and have found that the ATF routinely approves any tax stamp application that is filled out correctly.
Ranb