The M40 was a type of sniper rifle used by the United States military.
Al Crenshaw's weapon of choice was an M40 - during Operation Nightfall he had to use a Dragunov rifle instead, and claimed he would prefer to have been using his M40. (Nightfall)
The CTU standard issue sniper rifle was the PSG-1 Sniper Rifle rather than the M40A3, as the PSG-1 had enhanced accuracy over shorter ranges. (CTU Operations Manual)