How to rig weapon in 3Ds Max and attach to arms?

i imported ue4 default skeleton and a weapon to 3ds max and rigged weapon then added weapon to skeleton with “link constraints” and then made reload animation. i exported FBX file but in UE4 every bone has seprated (for example: magazine has an animation for itself and …) and weapon parts are not connected to each other.
in Unity i did this and everything were fine but in UE4 i have problems.

SO: “how to rig weapon(body,slider,trigger,…)?” AND “how to attach weapon rig to arms rig?”

​​​​​​​i have UE4 v4.22 and 3Ds Max 2019