It's damage profile is bad over range. It's RPM is slow so it's bad up close. It has small magazine and slow reload. You have to sacrifice perks to even make the gun workable in some manner.
What is your K/D? Why do you say the PK is the best gun in the game? My K/D is 2.30 and I'd never touch that thing.
It has the best range of the SMGs making it pretty much an AR with SMG mobility. just burst fire for long range kills (which you should know when or when not to go for anyway). Its reload is fine but yeah the magazine size is kinda small. that's why extended mags is the only attachment i use on it. I prefer having a bigger clip than faster reload since you can always fine moments to reload. plus extended mags saves you more than people realize (if you rush in a lot).
3.51 k/d, not that that matters. SPM is more important.