Ugh, totally misread that ><
In that case, while your’e not wrong, I’d say it might be easiest to export the TPose skeletal mesh and create your animations/poses in Maya or Blender or something (if you have it) with the gun in place, then import the new animations as needed.
I mostly only say that because as far as my experience goes, UE is not very intuitive with posing and animation, whereas with Maya or Blender you can do some very basic, simple rigging and get some really nice results a lot more easily.