Maybe try to hit the practice range before you start your sessions. I know you said you warm up your shots but standing still and shooting isn't really all that effective...try simulating battle situations. Maybe slide and empty a clip on a close Target then immediately switch to a AR and ping a couple of the distant targets. Or try landing slide shotguns on the close targets while alternating targets...1 left 1 right 1 center....this would help with realistic aim situations...bouncing from one to another...
If you just wanna get kills try dmg numbers on stacking...if you hear a shield crack and you've still got yours push hard unless you know he's got a pk or can't force the 1 v 1 don't try to thirst him if his teammates is pinging you
That's all I got