The UK regards percussion as a non shotgun for certification purposes. The USA also, so I am surprised by the $500 quotes as it should be possible to ship by common carrier postage.
Call one of the well known UK gun shops or possibly get a UK pal/aquaintance to bring it over in his checked baggage as his flight might be of less cost than the shipping cost and he would get a free trip to the states plus it is a much more secure shipping method as UK airports always hand carry guns onto the plane for no extra cost due to the security paranoia thats everwhere in the UK today.
US customs may be interested in the value though.
Martin
Airline personnel are generally not real smart regarding firearms, if it looks like a gun and went "bang" once upon a time they are likely to raise a stink when you go to check it in regardless of when it was made or what the law says.
Have documentation from both sides of its status (I would print out copies of the law) that it is not considered a firearm. As a note, had a friend purchase a flintlock rifle in the US and when he went to check in to fly back to Europe the airline at first refused to carry it.
After a hour of serious talking with everyone and a few calls to there headquarters in Europe to figure out it was OK they allowed it on. Leave your self plenty of time.
Anything made over 100yrs ago is duty free in the US