Valve's fault.
Snipers always kill with 1 Hit, except on Realism.
Same for Magnum, but this work in realism, too.
Maybe it's possible to make snipers as a 2-hit-kill weapon...
I'm not sure.
For me it seems, the damage of the rifles,smg etc. is in a good balance with the infected HP.
I think if there should be something done, then change the snipers...
btw: scout & awp have a very slow firing rate, so they'e a bit more balanced then the two others