Need assistance - first person weapon and arm animations not in sync

I had an animator create some first person animations for a shotgun: firing, reloading etc
He sent me gifs from within the editor after they were completed and they looked great:

For each animation I was sent 2 files ie. fire.fbx and fire_mannequin.fbx

The process for exporting these was:

1: Select the weapon bones and mesh.
2: Export selected, baking animation in appropriate selection range.
3: Select the mannequin bones and mesh
4: Export selected, baking animation, and checking off convert conforming dummies to bones

After sending these completed files to a developer however he’s unable to get them to appear correctly within UE4

As you can see the weapon and arm are not attached in sync.
Neither knows how to correct this or at what point something has gone wrong.
Any community assistance would be appreciated.


Gif of the issue:

Usually the weapon is attached to the arm’s hand socket and once this is done you still need to align it or rotate it in order for it to be fit in the correct place. Were these steps done?