Actually, both the AR-15 and the M-4 currently used by the armed forces are both derived from the M-16.  Although the M16 was introduced as a fully automatic weapon, it evolved through stages with selector switches permitting burst, full auto and/or semi-auto form, until it was determined that fully automatic fire is ineffective.  Almost every professional military rifle in the world is now semi-automatic without a switch to allow full auto or burst fire.  Full auto  consumes ammunition much too fast and reduces the number of rounds on target.  Just as with the M-1 Garand, and the BAR (which I was taught to fire semi-auto in spite of being fully automatic), semi-automatic fire permits effective tactical suppression.  The real problem with the M-16 family is that the round lacks adequate stopping power and range performance.  It is a misunderstanding to regard semi-auto rifles as inferior to full auto-rifles for tactical use.