Just picked up one of the carbon models with the picatinny gas block. I read the flattop is slightly higher so I need a taller front sight? Not a big deal, but am I supposed to buy a flip up "gas block" sight or "rail" sight. I'm not clear on the difference between the two and which style specifically I need for this gun. I'm assuming I can get whichever model I need and then swap out the actual sight pin or use an adjustable rear BUIS to compensate?