Excise tax is not imposed on the separate sale or
use of firearm component parts, including a frame
or receiver; however, the sale or use of firearms
in a knockdown condition (unassembled but complete
as to all component parts) is taxable. However,
under the GCA a frame or receiver is a firearm.[/url]
http://www.ttb.gov/fet/faetinfo.pdf
Technically, since the excise tax rates are different for a handgun
(10% vs 11% for a long arm), it would be difficult for the ATF to set the appropriate tax rate on a stripped receiver since it could be assembled into a handgun or long arm.