I had my plain barrel threaded by a member here. Put a M16A1 suppressor on it and hand tightened it. Came loose after a couple hundred rounds. Barrel vice is vital to getting it on tightly, and back off. My M16 vice doesn't quite fit the M&P15-22 barrel and left aluminum marks on the barrel. Bronze brush and CLP removed them is a minute -- don't panic if you see silver marks!
-- Chuck