To summarize:
10.5" barrel, with stock, no $200 tax stamp = Federal felony, $15,000 fine and/or 10 years in Federal pen (something like that).
10.5" barrel, no stock, gun declared as a pistol on initial 4473 = legal
10.5" barrel, stock, registered as SBR with $200 tax stamp = legal
10.5" barrel, permanently attached 5.5" flash hider (usually pinned and welded), stock or no stock = legal