Currently dont have the reputation to comment on the post itself to get more information. Will edit this answer once I do.

Can you show how the faces/edges of the weapon? As well as the normals. That currently in your setting under Geometry you have it to smooth normals. Which means there is a good chance that some faces are looping under and around the bottom part of the weapon from the grip to the middle of the weapon. So when it gets smoothed it will do some weird warping.