The full auto BCG in most premium ARs is in fact for durability and reliability.
There is a big difference between the AR15 and M4/M16 receivers. Semi-auto receivers are designed not to take the M4/M16 auto sear without significant machining. The most immediate difference between the the two receivers from the outside is that an M4/M16 has a third pin for the auto sear. Internally, you will notice that the rear of an AR15 receiver is solid. There is a big cut in the rear of an M4/M16 receiver for the auto sear to function. The other difference you will notice is that the full auto hammer has a hook to catch the auto sear. The semi-auto hammer has no hook. Part of the auto sear's job in closed bolt select fire weapons like the AK and M4 to prevent an out of battery firing that could blow up the receiver. There is also a high shelf in many ARs that prevents the use of an auto sear and drop in conversion devices.
Last edited by mboylan; December 22, 2011 at 09:30 PM.