Owning both the weapon and the barrel might be construed by the authorities as "constructive possession"
. You can have one or the other, but both might be problematic. BE CAREFUL, THE PENALTIES ARE STEEP. Once you have ATF permission to build the weapon, you are okay.
This may help discover how to go about it:
SBR FAQ from www.Quarterbore.com
EDIT: There are good resources at the BATFE web site. The government works for you (or should) and their web site has resources (forms, info, regs) that you should read and that will help you.