I tried to use a link to import a gun ak47 into ue4, and this is what it said: Failed to import ‘…/…/…/…/…/…/Users/ Computer/Downloads/BattleAR.FBX’. Failed to create asset ‘/Game/BattleAR’.
Please see Output Log for details.
I am assuming you downloaded a 3D model of an AK47? correct? is it a Static Mesh or a Skeletal mesh?
If so then you should see the output log for more details and post it here I will try to work it out with you, More in likely the model is not set up properly to be imported into UE4.