I believe it has to do with the file, I actually had managed to fix this error with the first weapon I had added in, but the second one I've made has had the error again. This time I can't see anything wrong with it at at all.
http://www.mediafire.com/download/5cdivlgl99hvl17/ExpandedBallisticArsenal.zip
Here is the zip file of what I have, the only added thing are two weapons: One is a PKM using the PKM image found in the game files, and the other is an FN FNC. I believe the FN FNC is what is having the error, as before adding it I was able to stop this error from happening.
Also, after saying the whole "cannont write to assets/mods" thing it will bring up another small window saying it cannot find the png for the FN FNC, but it very much exists.
As mentioned I had fixed this error with the first weapon I added, but since adding this second one it has returned.