Modded weapon magazine bug.
Noticed this so far with modded weapons of any type, can be a sniper, pistol, smg, ar, ect. Haven't seen it with minigun or RPG though.
When modded weapons are fired the magazine on the weapon will clip into the gun and rotate 90 degrees on it's side.
The bug is very frequent, but does happen at random
Has happened with weapons from several creators.
If you are confused about what I'm talking about, I'll link screenshots after the initial thread post because I haven't got to recreate the bug.
This bug happened to me a long time ago with some old weapon models, I'd think that's a problem with the gun model itself. Didn't see it ever again, and I think that shotgun you're using is one of the old weapon models I've used and were buggy, with the same problem as you has.
So my question is, does it happens with recently-uploaded weapons?