Another tip that goes along with this, which works for me, is learning exactly when to press R1 to fire off a shot.
When engaging moving targets from long distances, you'll notice that you won't have to much room for error due to the limited magnification of the Medium Scope (especially when you are aiming for headshots).
Instead of trying to move your crosshairs with the target, try to anticipate the targets next move. Surprisingly, when people are running around in the open they tend to just run in a straight line. Most likely, this is because people have a natural tendancy to get impatient, and take the quickest route when they spawn at a rear spawn point on a big level.
When you see a potential target, look at where the target is running. Once you think you figured out where he is going, line up your crosshairs about 10-20 meters in front of his running path. Once his head gets to the point to touching the red dot of your crosshairs, press the R1. Once you get the timing down, you'll score a headshot every time.
This is a tactic used by snipers/machine gunners in the military, and is sometimes referred to as "trap shooting" (not the sport, mind you). It's usually the best way to engage a target that is running left or right across your field of fire. The mindset is why constantly move your crosshairs to catch up to the target? When most of the time, with proper anticipation, the target will walk right into your crosshairs with little effort on your part.
Note: Although slightly a little more difficult, this can work on vehicles too (Especially on what I call "Doughnut Wonders". Anybody who has played respawns for a while knows how often the opposing team will get to your spawn and proceed to perform doughnuts right on your spawn area in an attempt to get more kills).
I use this all the time while playing Devil's Road sniping maps to deny the crossers their opportunity to cross.
Like I said before, people can always make excuses that sniping is easy in SOCOM 3, and in some aspects it is. If you can get to the point of nailing headshots on moving targets/vehicles with a rifle like the M-40 A1/L-96 AW, you are very skilled in my book.