It is not effected by stopping power unless it has a silencer. Without a silencer, it its a one hit kill from the lower chest up. With a silencer + SP, it does the same. So unless you are shoting people in the legs and arms, it should always be a one shot kill. I love the gun, nearly have 2500 kills with it.
I know I'm beating a dead horse here by bringing this up again, but I must have terrible luck with snipers or something. Occasionally, I'll get a 1-hit with the WA2000, but I can remember getting a 1-shot on the M21
ever. All these people saying "every sniper should be a 1-hit if you aim right"...what the hell are you doing that is so much better? The only guns I find dependable for 1-hits are the Intervention and .50 cal.
I use Stopping Power, and not normally a silencer, and for all the times I can remember, they were usually chest-shots.