Issue with FPV bones

Hello community,

I have an issue with the FPV skeleton from ADK. I made a custom mesh for a new weapon. I need ton build new animations but when I skin the model with hatchet FPV bones (properly) and i move the _jnt bone I can move it only one time. The second time it’s look like the bone is locked to the entire skeleton and all rotate arround and along the _jnt bone.
What I messed?

Thank you a lot