I'm having exactly the same problem, apparently only with one of my archguns (the Fluctus), which I have been re-leveling since getting a riven for it. My Imperator Vandal, also tater'd/gravimag'd/riven'd, doesn't seem to have the same issue and works fine.
You can't put forma on arch-gun heavy weapon
in General
Posted
I'm having exactly the same problem, apparently only with one of my archguns (the Fluctus), which I have been re-leveling since getting a riven for it. My Imperator Vandal, also tater'd/gravimag'd/riven'd, doesn't seem to have the same issue and works fine.