Xnormal and the UVS are separated, just like if they are different pieces, with hard edge... I'm starting to wondering if that hard edge is the problem
Can you post a picture of the UVs? If you're planning on switching out barrels, then you have to have a hard edge because it's two separate objects. If you have the UVs of the gun barrel laid out horizontally, but the attachment UVs are vertical, that could be an issue(though it shouldn't be in a synced workflow...) If…