Don't fly a plane without a joystick.
When playing US or with the L9 upgrade you can 2 shot people quickly by switching to pistol after putting a bullet into a non armoured type. The sniper will do 95% damage and the pistol the rest. The barret will be able to do this to any person, armoured or not. The chineese and MEC snipers are slightly less powerful so might not work, I never use them.
Log into your SF game and play a round. It upgrades your account to SF and means that you'll get 2 unlocks instead of 1 with level ups. Also remeber not ever level up gives you an unlock.
Always aim for the head, a lot of guns like the G36C will take up to 6 shots to kill against an armoured fo so you have to aim for the head and take your time to hit it. You will often get into a long ranged fight against someone and they will get through a hole clip and barely hit you and you can take your time to place just 1 shot on their head.
The gun sights are in fact miss-aligned with the skin. Many people do not know this. I don't know if it was lazy design or to create a sense of inaccuracy to the guns but the actually gun center point is miss aligned with every ironsight and scope. Some worse than others.