Binding Two Model Bones Each Other

Hey everyone. I’m working on CS:GO skins and I need some helps on animations. I have 2 objects which are weapon and hand models. Weapon has all the animations I need and I can play different ones with mixer. However hand model doesn’t have any animation but have bones. I noticed that weapon model also have hand bones. I tried to add arms bones to gun bones but result was awful. So how can I bind these two and animate at the same time? Thanks

m4a4_animations_2.glb (2.1 MB)
arms.glb (416.3 KB)