Useful tip which I have recently found: try to move (esp. in OSOK) to places where you can only be shot at by one person - try not to be out in the open when you can be attacked from all directions - back to the wall is quite useful.
EDIT: Unless you have a decent aim, you will never be able to beat a charging AR player head on who has seen you before you see him - good alternative - use shotgunners as meatshields or run to cover (if you can) - if you can't, try to take as much health as you can using the pistol, then, if you're still alive, finish off with the sniper/knife (i.e. may work against DES| players)
EDIT: Unless you have a decent aim, you will never be able to beat a charging AR player head on who has seen you before you see him - good alternative - use shotgunners as meatshields or run to cover (if you can) - if you can't, try to take as much health as you can using the pistol, then, if you're still alive, finish off with the sniper/knife (i.e. may work against DES| players)