Which is why the major manufacturers weld their flash suppressors to the barrel. If it's removable, it doesn't count.If you're going for a true M4 copy (14.5" step cut barrel), you need to fill out an ATF Form 1 and pay the $200 tax for it to be legal. With less than a 16" barrel it will be considered a Short Barreled Rifle/Title II weapon.
Unless you throw an aftermarket (longer) flash suppressor on it.