I have 2 complete AR's in 5.56 caliber, one a 14.5 inch barrel and one 16 inch barrel. I own no other uppers or lowers. I want to build a complete upper in 300 Blackout with a 12 inch barrel, which will make it a SBR under NFA regs. My question is, if I find the particular 12 inch barrel that I want online, can I legally purchase it and keep it in my house with my other two AR's until I get ready to file my paperwork and get the lower purchased and engraved. The make and length barrel I want is hard to come by right now, and if I find one, I would like to be able to purchase it as soon as I can by law. Many thanks for any help.