The mod I'm using is Vanilla Firearms Expansion. I just went to the mod comments section it seems like a lot of people are reporting the same problem. It should be the mod is incompatible with 41.78.8
41.78.8 broke the Firearm texture, noticeable on M16 Assault Rifle and JS-2000 shotgun. When I switch back to Stable version it seems to be OK. Pls look into it.