Lo Syko and welcome to KM.

Each gun and rifle has differant attributes when it comes to firing. With most of them its always best to aim lower as the gun will move up when you are firing, also shoot in short bursts rather than keeping your finger on the trigger.. this will improve your accuracy.

Also practise with the differant weapons as much as you can, you will then notice that each react in a differant way..

Some members have made posts that can help you out, i would have a read, there is some great advice from some good players in there..
Foxxs Tip Shack
SMGs for Dummies
Glocks
AK47