Adjustable blocks on AR-15's are more common with supressor, but usually not necessary otherwise. You can usually tune the system with different weight bolts, buffers, and springs.
Actually, it's very easy.
Originally Posted by Powderman
Also--if you have a front sight/gas block that is pinned in place, you're going to have a devil of a time getting it off. Those pins are swaged in under tremendous pressure.