The M24 is the 2nd most powerful rifle, with a 100% chance of 2-hit killing them if they're at full HP, not 3.
If their HP is below 90 (I think it's 90) it's a 1-shot kill. And you can blow someone's brains out from all the way across Snow Valley (if you have somebody use nofog and shoot you from one end of the map while youre at the exact opposite end) it'll register the hit right in their face. I use the M24 and I've gotten pretty fast at headshotting people before they can shoot me with their nub l9. ;D