Ok, I'll keep this simple for the people who dont like the technical bullshit.
The M-16 is the standard assault rifle of the U.S. military, and has been the standard for over 45 years. It is well known for its new design, accuracy, and reliability on the field. The M16 was made to be able to adapt to any given situation at the moment, and as a result it can accept a wide range of accessories and extra equipment.
Because of the facts above, the M-16 has been adopted by many other countries(UK,Iraq,Italy,etc.).
"The AK-47 is the best assault rifle in the world"......(Shoots target) Damn! I missed again!(trades for M-16).