Mostly interchangeable, BUT...
M16 fire control components (trigger, disconnector, selector, hammer, sear, and bolt carrier) should not be used in an AR15, nor should they even be in your possession if you own an AR15. Mere possesion of those six parts in conjunction with an AR15 can be prosecuted as an unregistered machinegun. They don't have to be installed in the gun, nor do they even have to be in the same location as the gun. Additionally, while there are some upper & barrels assemblies shorter than 16", they should only be used on one of the following:
a.) an AR15 pistol (no stock)
b.) a registered AR15 SBR (short barreled rifle)
c.) a registered AR15 AOW (any other weapon - no stock, vertical foregrip)
d.) a regisered machinegun/M16