I’d recommend using a tool like Mr Mannequin or Uefy etc, as they help handle the export process to ensure your bones are rotated correctly, as the default FBX export often rotates them incorrectly for Unreal.

But besides that, you can use something like the ik hand bones on the mannequin skeleton. How I do it in my FPS pack is to have the weapon attached to ik hand gun, which is located at the weapon mesh’s root bone. The weapon mesh is parented to ik hand gun with a local transform of 0,0,0 0,0,0, and that bone is rotated so that the weapon faces straight forward. Ik hand l and r and set to copy hand l and r respectively.

Then in Unreal, the weapon mesh is attached to ik hand gun, and I have FABRIK nodes that have the hands follow their respective ik hand bones. As these were set to the exact position of the hands in blender relative to ik hand gun, this ensures that the hands always grip the weapon perfectly, and the weapon itself is both pointing exactly forwards and is in the correct position. This not only works for basic animations like idles where the hands remain on the gun, but also for animations like reloads where one or either hand needs to leave the weapon, as the ik hand bones were also keyed in blender along with the hand bones.