Then to get it into the U.S. the Customs Service could get you embroiled in a whole 'nuther set of complications.
I don't think the U.S. cares at all about barrels, do they?
My understanding is that in Germany, the barrel is the regulated item instead of the receiver like in the US. If that's the case I assume you won't be able to buy one there much less bring it into the US.