Why does the AR-15 hammer have small notch in the hammer at the top, and the m-16 hammer does not?
It’s to catch the firing pin in the event that the disconnector failed. It prevents full auto fire. It serves no purpose however if you have an M-16 carrier, or a carrier with a fully shrouded firing pin. The ramped AR carriers expose the back of the firing pin. This is where the notch acts as a safe guard against allowing the bolt to return to battery without the hammer being cocked.
If you half cycle your AR with the charging handle you can simulate a disconnect failure.
Thank you!