The M40A3 was first created in 1996 by Remington. The M40A3 was based off of the Remington 700 sniper rifle, but the only difference the M40A3 has over the Remington 700 is it has more accuracy but less stopping power. It is used worldwide but mostly by the United States and its NATO allies.
The M40A3 appears in CoD 4 Modern Warfare in multiplayer. It is part of the default sniper class and its only attatchment is the ACOG scope. It has the unique ability to kill an enemy with a single shot anywhere when ACOG and stopping power are used.