Weapon and arm animations 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: https://cdn.discordapp.com/attachments/343538120542650369/389256563870662656/fire.gif

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 https://cdn.discordapp.com/attachments/344161523863650305/406231935434752001/unknown.png https://cdn.discordapp.com/attachments/344161523863650305/406232249462423572/unknown.png

To add a bit more context, the animation was done with the UE4 mannequin in 3DS Max, not our specific arms/hands.
In addition, in-engine the scale of the weapon had to be increased to 2.5 because apparently at 1.0 the model was too small.

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

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.