It just takes practice, lots and lots of practice. Good aim basically boils down to:
1) Understanding the recoil pattern of the weapon. Firing range helps a lot with this. Once you understand the recoil you can counter it while you fire to keep the bullets going as straight as possible.
2) Understanding your engagement distance and bullet drop off. Firing range helps with this as well. Knowing how far your enemies are and aiming higher to counteract the drop off so they land where you want.
3) Predicting player movements. If someone is running to your left and they jump, you can guesstimate where they will land and aim there instead of aiming directly on them. This gives you a split second to predict, aim, and shoot.
4) If you're up close, aim right below peoples necks, recoil makes your weapon kick upwards and you can get some headshots in.
That being said, you can have the best aim in the world and still consistently die, because like what others have said, POSITIONING IS KEY! You can have potato aim like me, and still pull off good plays based solely on your positioning. Your positioning should:
1) Make it harder for the enemies to hit you.
2) Make it easier for you to hit them.
3) Give you an opportunity to disengage if the fight goes bad / another party shows up. HAVE AN EXIT ROUTE!
Having plenty of cover to mask most of your body is key. Don't be afraid to peek out, shoot, then take cover. The harder you are to hit, the less damage you take.
Before engaging in any fight, consider these:
1) What weapons do I have and how close should I be to land the shots more accurately? Light ammo weapons up close aren't as effective as say, a shotgun to the face.
2) Is there any cover that allows me to engage at that distance?
3) Where are my teammates?
4) Where could other enemies attack from if I do engage?
5) How do I get out if the fight isn't working?
Hopefully these tips help, but ultimately it just comes down to practice, practice, practice. Getting a feel for the weapons and how players can move will become muscle memory and your aim will improve over time.
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