As long as you buy the parts from them, Bushmaster doesn't charge for installation, unless they've very recently changed their policy. What you want to do is perfectly legal. As you're fortunate enough to have a pre-ban lower, a Phantom flash suppressor permanently affixed to a 14.5" M4 barrel does meet the 16" minimum legal length (mine has this configuration.) A standard A2 is too short. Kurt's Kustom makes an elongated A2 style if you want the "look" of a standard A2, but I myself liked the Phantom. Plus, Bushmaster will also do the permanent installation.