To adjust hands and a weapon with the IK system, you should open the adjustment scene [Tools -> USK -> Adjust], then place the weapon or a few to the Adjustment script, go to the Play Mode and choice the weapon there.
You can adjust the position and rotation of the weapon in the hands.
These slots are needed to use weapons in various views and with different characters.
For each view and each character you can create own slot and use it separately.
This's the main part of the weapon adjustment. Here you can adjust the character's hands for the different modes.
It's the usual weapon state when a character is being idle, walking or running.
Move and rotate the red cubes to adjust the hands. If you toggle the "Disable IK in normal state" , the IK system won't be active in this state.
It's the state when character is aiming.
Use the blue cubes to adjust the hands.
It's the state when character is near a wall or any tall object.
Use the yellow cubes to adjust the hands and also you should edit the "Collider to check".
This state will be useful for the third-person view if in the crouch sate your character's hands are in the body.
Use the pink cubes to adjust the hands. If you toggle the "Disable IK in crouch state" , the IK system won't be active in this state.
Move the green spheres to adjust the direction of the elbows.
There you can adjust the fingers rotation offset as you need.