I would buy your AP4 right now if it had an 18" Government Profile mid-length gas system chrome lined. The 16" cuts the .308 velocity down to only 92FPS above what a 7.62x39 AK47 round does out of the same 16" barrel.
Do it right, chrome lined, government profile, 18", mid-length gas system, usable bayonet mount, optional flash hider, A3 upper, foward assist, dust cover, six position stock, flip up BUIS, and I will sell every damn AR I own and get only that type of set-up. It would be the lightest .308 mil spec style weapon system capable of engaging man sized targets to 800 meters. Get rid of the 16", and leave the bull barrel systems for the stainless 24" series, this would be a field deployable lightweight long range battle rifle.