If a target is moving, make sure to lead it. If you are not making shots, 
try to lead your target more. Also, if you are a skilled sniper, try using 
the M40A3 over the M21. It has a slower reload time and rate of fire, but it 
has enough damage to kill a target in one shot instead of two or three. It 
also allows you to be less noticeable with less rounds being fired so 
someone else cannot follow the tracer shot back to you. A good setup for the 
sniper is the Claymore Perk to alert you when an enemy is trying to close in 
on your rear, and either the Sleight Of Hand or Stopping Power, then Iron 
Lungs to give you more time to line up your shots.