I have a question about SBRs. I just bought a Kahr Arms 1927A1 Thompson with the 16.5" barrel. I'd like to buy a GI 10.5" barrel and legally have it converted to the more original looking Thompson carbine. So my question is, do I have to have the SBR paperwork done and approved before I buy the barrel or can I have the rifle and the barrel and (barrel not installed) before I get the tax stamp approved?