Sadly it's the game mechanics at fault here. The PW7A2 smg is a beast and is a laser beam even at long range, it needs a definite nerf.
And getting killed behind cover, THAT would be DICE's terrible net code, hit reg, tick rate, whatever you want to blame there. I personally blame their corporate people here, they probably don't want to spend the money for more servers to help with this, and figure people will stop complaining about it after a while, or are telling the Devs to fix it without spending extra money.