I'm an excellent shot with over 550 hours.
The only thing in the game which is highly unique in regards to aiming is how much stamina and movement dominate accuracy.
Basically, conserved stamina is an accuracy buff while movement is an accuracy debuff. Whoever has the most stamina and is moving the least tends to win the encounter. All other considerations come secondary to this understanding.
Thus the key to getting shots on target and dropping the enemy is maximizing stamina and minimizing movement during a shot.
Of course this is really hard to achieve since like all FPS games the gameplay rewards aggression/speed and movement. So what do you do?
Always try and halt and steady, even for a split second, in order to recover a semblance of precision before a shot. Remember that slow beats fast. Remember that pausing to regain stamina, even in very close encounters, helps you produce an overmatch. Know that the one who sprints is usually already dead.
Obviously this isn't enough. You also need to recognize the limitations of the weapons. I suggest stability after the first shot is critical.
Most of us can make the first shot, but rarely does that first shot drop the target.
Unless you brain the guy, opponents take two-plus rounds (or two-plus bleed time) to kill. For me, only a few guns can keep a decent amount of stability after two rounds while also putting rounds out fast enough to waste a guy, even on single shot, even at close range.
And if you just ran up to a wall and acquired a target, better hope you have one of the two guns below!
All the pistols can make multiple shots at almost any stamina or movement state. And the ironsight M4 with front grip can do it too (this is the best weapon in the game for continuous fighting ability at all ranges). Pistols and the M4 both recover faster from the bounce of the previous shot and retain precision even if exhausted than all other weapons (even the Psssshhhhhh). Pistols and the M4 also benefit from excellent sites.
You should recognize that weapons like all AKs are challenging to make shots with at all ranges, due to their high recoil and poor sights, and especially challenging when put on full auto.
Full auto is all but useless at range and almost never effective as a hose even at close range. Too much kick. Too much blinding down the sights. Too hard to judge shot placement after the first handful of rounds.
Where full auto is useful is when 1) you expect CQB, 2) you use full auto to place a burst into your target.
Burst fire at close range (5-10 feet max at 3-4 bullets per) is highly effective, but you need to learn the ROF and how quickly the gun jumps per shot. Again, the ironsite M4 with forward grip is the king here. Epic rate of fire, the best stability in the game, good sites and very low kick compared to all other weapons. You can go FA with this weapon out to 20 feet or so and make three/four shot kills.
Also crouch > prone (because you can move without losing stamina while crouched).
And finally. A little trick. The game seems to treat W/S movement different from A/S. Only forward or backwards movement soaks stamina. But you can sprint while sidestepping. Also spinning without W/S movement doesn't seem to effect accuracy at all.
You can slice the pie (sidestep around a target, or corner, etc) while holding your zoom/stabilize/sprint button and adjusting aim. As long as you only tap the forward and back buttons to make minor adjustments, you can maintain zoom, sprint (as a sidestep), and stability. Toss a lean in, too. It is brutal. You will turn corners locked in at speed and suffer very little loss of accuracy.
All this assumes your your rig can drive the game at a playable pace.
Mouse is at .34 and smoothing on.