EDIT: This as been fixed for a while now…
This is what I saw when I spawned as Slugger (shotgun engie):
Then after fire the gun once:
The highlights in the first pic are the same as those seen when firing the gun (muzzle flash reflections), but when the gun comes out they are ON by default and stay stuck ON until the gun is fired. I assume they disappear at that point because the firing animation cycles that texture ON then OFF.
If you swap to another weapon or the stickies and come back to the SG, the effect returns until you fire again.