The M16A4 is an assault rifle that comes from the M16 family, which branches out to the CAR and M4 families. Using the fairly strong 5.56 round, this is one of the "automatic snipers" in the M16 family. It has a fairly long barrel for great accuracy and decent effective range. If equipped with a telescopic scope, it has the potential of being one of the best medium-far weapons in the game. However, its major shortcomings are the lack of automatic fire and the low control. Not a lot of people get this gun because of this, and they opt for a lighter automatic M4 instead.
Some people have used a base M4 model, replaced the barrel for a M16-length one for accuracy, and added a M16 fixed stock for more control, making it an automatic substitute for the M16A4.