I did tutorial two times, but still it’s not clear for me.
There no percenages, how I should understand hit probability? Moreover sometimes I see enemy animation, like head swinging, does game take it in account when I press fire?
I also don’t understand second external circle, if hitting area is inner what purpose for external?
50% of shots are guaranteed in the small circle. 100% of shots are guaranteed in the big circle. I’m pretty sure that the idle animation updates with aiming, at least it seems to work for me. For example a lot of the times i’ll back out of the manual aim and wait for a cloaking triton to show it’s torso before shooting it.
Yes, the idle animation impacts where the shot lands. Not only that, but the target continues to animate in slow motion after the shot is fired.
Anyway, the system is actually quite involved. If you are interested, it is covered extensively in the guide I was working on, with explanations from other players and the devs:
So, from what I understand if both circles fully cover specific part of body I have a 100% hit and that’s my way to win 
Actually, that’s not quite right.
100% of your shots are guaranteed to land within the outer circle. BUT the inner circle doesn’t guarantee that half of your shots will land here - it merely indicates that there’s a 50% chance that each shot you fire will pass through the inner circle.
But that’s only a chance, not a guarantee. I refer you to the classic origin story of Murphy’s Law - Murphy was a maths teacher who wanted to teach probability to his students, so he had each of them butter a side of bread and throw it into the air, to demonstrate the probability that 50% of them would land butter-side up. 29 out of 30 landed butter side down. The other one stuck to the ceiling. 
Not necessarily, because this is a ‘proper’ ballistic system. So if there is a small gap somewhere, the bullet might pass through that.
We also believe that there is also a bug which means that there’s a million-to-one chance that a completely covered large body part will somehow get missed - and in the immortal words of Terry Pratchett: “Million to one chances come up nine times out of every ten.” 
Overwhelmingly, yes, provided that you are really completely covering the body part - as MichaelIgnotus points out there must not be any “gaps” (as, for example, happens with Arthron launcher arms).
Then there is the idle animation - the target actually keeps moving in slow motion as the shot is fired, which has a noticeable impact with long burst weapons (like the heavy machine gun). This is intended behavior to make the “snapshot” useful.
Also, with Rage Burst (Heavy Lvl7 skill) the target will often recoil from the first impact and slightly change position.
Finally, it can happen sometimes that the ‘hit box’ of an obstacle is slightly bigger than the obstacle itself (particularly some irregularly shaped obstacles like rock outcroppings).
Are you sure, that 50% is chance to hit inside the circle, but not the point on that circle? As I understand, when UnstableVoltage explain that system, 50% is chance to hit exactly circle. And chance to hit outer circle and center point are equal and near null
UV clarified in some other post that the 50% chance was calculated for the outer edge of the inner circle, not the centre.
So you have a 50% chance that each shot will distribute somewhere around the halfway-point of the radius of the outer circle (ie. the outer edge of the inner circle) - but it is still only a 50-50 chance, not a guarantee that 50% of your shots will land there.
I think most of the others have covered what the mechanics are here, but just wanted to add something. In a previous build I can’t remember which tbh, you could take a second shot while Crabbies were slow mo-ing putting their sheild back in place after recoiling from first shot. I really enjoyed that element, and not sure why it was removed.
Yeah, there is no-roll against the target. The system distributes bullets along the UI using bell-curve - meaning bullets are less likely to land in the exact centre, and and the extreme outer circle. Because if that, centering aim at the body part you want to hit the most, is not always the best way to do it.
About 50% should land within inner circle, and about 50% should land within outer circle - it is not guaranteed to happen though.
When enemy hit, he moves, which means he can cover soft spots with armored path, or weave out of the way, so in burst shot not every bullet will have the same chance to hit.
Whenever you use snapshot, or manual shot this system works the same. However, snapshot locks to enemies centre of mass - as he moves the aim will adjust even in the middle of the burst shot. Manual aim on the other hand aims at the point in space - so if enemy moves out of the way the aim will not adjust to compensate. So depending on distance, enemy type and weapon used snapshot or manual shot might be more desirable to use.
That’s interesting. I’ve almost never used the snapshot option feeling it was inferior in all situations.