The AK-47 is the most versatile weapon of choice for the Terrorist teams. With it's one-shot capability, it makes an excellent entry-fragging tool, and with high damage it is useful everywhere else too!
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The AK-47 is famously a 1-shot headshot, which is one of the reasons it is so beloved by Counter-Strike community. Additionally though, it has a high damage output and armour penetration, allowing it to kill in 3-4 body shots. This means that a short burst can be very deadly if you are accurate enough.
Learn the spray patterns
Trace the loop on the right for a perfect spray!
Uncontrolled spray (courtesy: twowordbird)
The AK-47 is your bread and butter. It is imperative that while learning CS:GO you learn the spray pattern (by dragging your mouse according to the second diagram above), so that you are prepared for any encounter. To start off with, join deathmatch servers or practice against a wall, by bursting 3-5 bullets and trying to get as tight of a grouping as possible.
See it in action!
The AK-47 can be used at any range. For short range battles it is preferrable to use it in full-auto, for medium range you can vary your shooting from full-auto to burst firing to tapping. Long range battles should be taken with burst fire or tapping. It should also be noted that the AK-47 also has a quick recoil reset, meaning that tapping quickly will not incur too large of an accuracy penalty.