As I said, whether or not it's overpowered is a different discussion. It is a top tier weapon that most people run (myself included) and for good reason. So it might need a nerf. I agree that the reload cancelling needs to be fixed. The drawback of the gun is supposed to be that it's lever action. Bypassing it makes it overpowered.
But if we are talking about one shot potential, the gun cannot do this if you have level 3 shield. This is shoddy netcoding and should not be factored into balancing weapons.
Are you sure it was 197 and not 187? If the game is rounding damage numbers down, then a level 3 (purple) helmet will reduce Kraber headshot damage to 187 (rounded from 187.5).
Other than that I don't know, according to the damage numbers it shouldn't happen. Do you have a video or screenshot of the 197 damage?