"Constructive possession" like having an auto sear is what they'll hassle/arrest/imprison you for.

Stick with 16" - the velocity of the tiny little light weight .223 rounds drops to nothing out of a shorter barrel. If you would ever have to shoot something bigger than a poodle you'd have issues. The .223 round is only dramatic when it tumbles in flesh. This is generally consided to happen above 2700 ft. per second. Out of an 11.5 inch barrel M855 is below that threshold at about 30-45 feet. That's right - at a distance shorter than you can throw your short barreled rifle it's not able to tumble. With M193 ammo that distance stretches all the way out to 120 feet. Dang that's close!

Shoot that same M855 ammo out of a more manly sized barrel of 16" and you're looking at 250-300 feet and the M193 is good for 400 to 500 feet. These distances are typical engagement distances for urban warfare. If you need more distance step up to a 20" weapon and you're looking at 500 and 650 feet.

The reason our military uses longer barrels isn't for looks and you shouldn't buy a weapon with a shorter barrel for looks unless you're just looking to make basketball sized muzzle flashes. If you want a weapon with an illegally short barrel that really puts out a blast AND is still lethal out to range get something other than a .223 ... that way when your sitting in federal prison (it's a federal gun law you're in violation of) you'll have more fond memories.