All of the sniper rifles do 70 damage (50 damage when silenced), it's just that they get different damage multipliers based on where you hit. Players have 100 health in regular modes, and 30 health in hardcore modes.
http://denkirson.xanga.com/715966769/modern-warfare-2/
The Barrett and the Intervention are basically the same thing. Use them with Stopping Power and you'll always get a 1 hit kill unless you hit the arms or legs or go through a surface. Never, ever use these with a silencer.
Don't use the WA2000 with Stopping Power, unless you are using the silencer with it. The WA2000 is always an upper body 1-hit kill, and a lower body 2-hit kill. The silencer will make it a 2-hit kill regardless of where you hit, but Stopping Power will boost the damage back up to 70 again.
The M21 should not be used with Stopping Power. It is pretty much always a 2-hit kill, silencer or not. Use it silenced with Cold Blooded or something.
A multiplier of 1.5 will boost 70 to 105, and 50 to 70. A multiplier of 1.1 will boost 70 to 77, and 50 to 55. Stopping Power multiplies bullet weapon damage (and individual shotgun pellet damage) by 1.4. So a multiplier of 1.1 with Stopping Power will be a multiplier of 1.5, enough to get a 1-shot kill if your weapon does 70 damage.
Those in-game weapons statistics flat out lie to you. Use that chart I linked to instead.