Simplify the following text for a Grade 6 native English-speaking person.
Original: The M16 ( more formally Rifle , Caliber 5.56 mm , M16 ) is the United States military designation for the AR-15 rifle .
Simplified: The M16 is an assault rifle used by the United States since the Vietnam War in 1963 , based on the AR-15 . Since 1975 , the M16 has been used by many different countries .