First of all, if your rifle barrel isn't OVER 16", then it is illegal unless it is on a lower that's either a machinegun or an SBR.
Having said that, you can legally make a short barrel longer by *permanently attaching* a muzzle extension. Once permanently attached, the muzzle extension will count as part of the legal barrel length.
A 14.5" barrel with a permanently attached A2 FS is only 15.85" long, but I'd bet that your FS isn't even permanently attached. In order to be legal, you need to have an extended muzzle device (a Phantom is your best choice, but the Vortex or Kurt's Kuston A3 would work also) installed, and then permanently attached. The preferred method is the "blind pin & weld" method, as it is least harmful to the barrel.
Kurt at KKF can do this for you, as can many AR smiths.
-Troy