Hey @SebastienMULLER! Welcome to the forums!
I think I understand what is happening. The rotation of the weapon is still getting rotation from the constraint. Personally I’ve never done it this way, typically I’d have the weapon be a separate actor. Then to attach it, you spawn the actor and attach it to the socket on the skeletal mesh- that being said, with this animation, that would come with its own problems.
What I’m thinking is- have a second mesh of the weapon. Then, as part of the animation, make the weapon in your hand invisible, but have the backup teleport to the spot where it would be lodged in the ground.
It’s actually a very similar idea to how sheathed weapons work in many games. The sheathed weapon on the back or hip and the weapon in the hand switch visibility as part of a sheath/unsheathe animation.
Hope that helps!