The whole game is based around bullet randomness. That mean's in some situations, even when you are aiming perfectly at the head, you will not hit him.

The weapons in CS:GO are designed in a way to be effective at certain ranges:

Pistols/smgs/shotguns: Close range Riffles: close and medium range Scoped weapons: all common ranges

This is reflected in the price of the weapon.

There are three different levels, which can increase randomness:

1.) First bullet inaccuracy (You stand perfectly still, aim at the head and shoot once)

2.) Movement inaccuracy (Shooting while moving)

3.) Recoil inaccuracy (Repeately shooting fast)

For point 1. you should consider what I said above. Each weapon is designed to be effective at different ranges.

For point 2. you should know that in CS:GO there is an acceleration and deacceleration.

Each weapon has a different movement speed and when you start running, you slowly built up speed until you have reached your maximum speed. The same goes, when you let go of your movement keys. It slowly goes down until it reaches 0, this is the point, where your bullets are accurate again. There is a technique though, which is called counter strafing and allows you to stop your movement instantly. You should do your research on how to use it and practice using it right away, as it is a huge advantage to know that technique, since you can start shooting faster and more accurate than people who don't use this.

As for point 3, you should know, that after you fire a weapon, there is a small time period, where your bullets get more inaccurate. You basically, have to shot, wait until your gun regains it's accuracy and then fire again.

In general randomness effect's the game more in long distances battles (because the player modles are smaller) than it does in close and medium range battles. That's why you will see people running around with pistols and holding their attack button (spraying) in close and medium ranges.