Unsure why gun and arm animations are out of sync

Hey,
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.

Thanks

edit: gif of the issue from within game: https://im5.ezgif.com/tmp/ezgif-5-478d56953c.gif

In blender a common problem for people is that they forget to have the scale of their objects all set to 1. But Im not sure whats happening there.

I’ve attached a gif of the issue to the original post