i am like level 39 or 40, just got the overkill perk, and still love the m16 gun as my main weapon with either stopping power or deep penetration or steady aim, i shoot from the hip pretty good with that perk enabled.
if you have a logi with thumb buttons USE THEM!! you must use the aiming down the sight, so i assign what is the thumb back button on my mouse to aim down sights, and when i release it the sights go away so i can control it at will, my favorite way.
i assign the thumb forward button to reload.
i assign the mouse wheel up and down to switch weapons, and when i press down the mouse wheel i have that as jump. i have my primary fire as my left mouse button that is standard, but i have going prone as my right mouse button.
learn the maps for sure, dont charge out like a maniac this game u die too easy to do that all the time.
i tend to crouch a lot and you aim better, only thing is when you get into a close quarters fight, the habit to crouch is there and then you become a stationary target while he hops around. i personally dont need the red dot sight, i can pop off long distance shots with the ironsights on the m16. but i can see how it helps for many people, so use it.
when you play against good players a lot of times they are running around with their gun aiming at near head level to get a headshot, if u have good reaction time and can react first when you run into somebody going around the corner, crouch and shoot, their shot will go over your head and your shot goes right in their chest. of course u need to have cat like reflexes like me