Guys... all snipers do 70 damage.. they just have different damage multipliers!
The Barret and Intervention have 1.5x for the head, neck, and chest. They have a 1.1x for the torso (this is where stopping power helps). The limbs and pelvis are all 1.0x (stopping power doesn't help).
The Wa2000 has 1.5x for the head, neck, and chest. Everything else is 1.0x (stopping power doesn't help).
The M21 has 1.5x for the head and neck. The chest is 1.1x (this is where stopping power helps). Everything else is 1.0x (stopping power doesn't help).
The real differences between the sniper rifles is that they have different recoil, handling, reload time, scope in/out time, and just the way they look and stuff.
***Silencers reduce the damage of all sniper rifles by 20 damage, making every bullet do 50 damage, instead of 70!***
The M21 is the easiest to use, since you can empty your clip with minimal recoil. The Wa2000 benefits from not needing stopping power because then you can use a different 2nd tier perk.
It's really just about personal preference! I like all of them, but use the Intervention and M21 (but still use the Barret from time to time!)