I have recently imported percussion guns from England, South Africa and New Zealand.
All the guns were c.1840 to 1860. The shipping end understood US law better than our U.S. Customs and Border Protection folks. As Steve and Toby mentioned, no ATF Form 6 is required. Be sure that the shipper dates the gun on every form possible. Don't rely on the fact that it's a percussion or flint ignition system - Customs won't have any idea what those are.
There's a very helpful document on the Customs and Border Protection website - Requirements for importing new or antique firearms/ammunition.
