Hello,
I’m making an FPS and the character can see his arms an legs, I want the hand to follow a point, a little bit like in Surgeon simulator : the hand follows the mouse.
The problem is that I need the shoulder to stay in position and the elbow to automatically move to the only available position (the three bone are a triangle).
I made a drawing to explain it.
So is there a way to rotate the bone to have the hand at the target point ?
I can’t give you a tutorial because I’m not 100% certain about it myself but my suggestion would be to look into IK or Inverse Kinematics. Here’s the unreal documentation for it: IK Setups | Unreal Engine Documentation 
As leon said, you can simply use IK for this. Have a look at the TwoBoneIK blueprint component.
Perfect !
I looked at it in the content sample but I thought it was for reversing the animation ^^.
Thank you very much guys, relly usefull !
EDIT : I don’t understand what does the Joint Target Location value do in the Two Bone IK node ?
Happy to help! 
Make sure to ask if you have more questions.
I don’t understand what does the Joint Target Location value do in the Two Bone IK node.
Hey Thomson_Skelington -
I am going to link you over to AnswerHub, there is a post which may directly answer you question. How do I attach IK to another bone in the same skeleton? - Character & Animation - Unreal Engine Forums](How do I attach IK to another bone in the same skeleton? - Character & Animation - Unreal Engine Forums)
Eric Ketchum
Mhm… I still don’t get want it does… Explain it again please 
The Joint Target is responsible for keeping the chain rotation from going whacky, basically.
Check out this link What is "Joint Target" in the IK node? - Character & Animation - Epic Developer Community Forums and this one Does UE4 have pole vectors for IK? - Character & Animation - Epic Developer Community Forums
You basically just need to set it to be the middle bone (i.e., if you have thigh -> knee -> foot, it’s the knee bone).