It's not all about killing people on 12. You have to be in the mindset to get better. Having the "Getting kills" attitude in your head won't make you any better in a DM server, because you'll find cheap ways like camping a corner or spraying like an idiot when you're behind someone.

You have to mentally think "Okay, I want to get better"
You start looking at your aim. If you notice you move your mouse past the enemy, lower your sensitivity. If you notice that you can't quite reach the enemy sometimes, then raise your sensitivity.

Another thing I like to do in DM is having my crouch key unbound. As much as you see people do it, you don't want to crouch and spray. It just leaves you vulnerable.

Also, always go for headshots. That's the best "target practice" for me.

Experiment with different styles of shooting. Find one that suits you that gets you kills. What I mean is,
when an enemy is long range do you single burst? do you 2 burst?
when an enemy is mid range do you 2 burst, do you spray?
when an enemy is short range, do you go for the headshot, do you spray, do you 2 burst?
etc.

DM is all about working on your accuracy. Focus!! Don't jsut run around like an idiot. Won't work as effectively

I'd gladly help you if I could play xP