Yes i used it many times in gms to do all 3 champions at once
Its one of my main weapons.
The slow only procs after landing shots.
The gun itself isnt shooting slow bullets its regular scoutriffle bullets.
So there is a bug somewhere.
Either from it being able to be anti barrier for so long (even with radiant it worked)
Or from it not being able to be a barrier weapon anymore.
Or 3rd possibility : they made this change on purpose but didnt put it in the patchnote