Hey, so I was having this same issue where on keyboard and mouse if I was aimed in and shooting at nothing there would be no issue but if I was shooting at someone (Even hip firing) My aim would lock up like crazy. Almost like there was an invisible box around the person I was shooting at aiming/hip firing would just come to a halt. I tried every single setting I could find online. Everything from controller aim slowdown and auto aim assist(even on mouse and key) to uniform aiming and many other things. Finally found the issue in my case, i just upgrade pcs and thats when this issue begun.
If you're using a mouse (in my case Corsair m65) or probably any mouse that has the side button that is a "feature" to slow down your aim (called the sniper side mouse button on mine) My game was somehow getting the input to activate that even without pressing the button. So I went and downloaded the Corsair software(did not have to do this on my old pc) and set that side mouse button to the same DPI as my regular mouse DPI. That instantly solved my issue, but again if you don't have a side mouse button I'm not sure what the issue would be. But hopefully any other corsair M series(or mice with the same type of button) see this and it helps.
If you're having the same issue I had then you know it's nothing to do with PC performance and is very obviously an aim related issue. Pretty frustrating to read through all those threads and all these people saying just turn g-sync or uniform aiming on and off etc...
