It seems Veist Stinger does not want to proc on Trachinus or Lionfish-4FR. I did some extensive testing to make sure it wasn't just "bad luck" and even after hundreds of slugs (both body or precision) and fusion bursts (only body, can't crit) the trait is not activating. I did additional testing on other Veist weapons (I still happen to have) such as Krait, Whistler's Whim, Jararaca-3SR, Irukandji, Prolonged Engagement, Fortunate Star (revised), Mistral Lift, Scintillation, Fire and Forget, and Boomslang-4FR. All of these other weapons are able to proc Veist Stinger in a relatively reliable manner. Other than the fact that Special and Heavy weapons struggle to activate Veist Stinger sometimes, it would work within the reserve pool of ammo. However, even after multiple reloads and tons of ammo dumping in enemies, Trachinus and Lionfish-4FR just do not activate. So anyone wanting to min-max Trachinus, just use Tenacity.