Hello again,
This is just my thoughts on what I think would be a fun Psyker specific gun to have in the game.
The weapons special action is to juice up the gun with witch power so the next shot(s) will have additional affects. what type of extra wizard effect depends on what that Psyker player has selected as a Blitz (Brain Rupture, 40k Force Lightning, or Wizard Knives).
I think simple stuff, like greater armor piercing, imparting electrocution on hit, and homing bullets would work well. Think something like the empowered gun dishes out lesser blitzes with bullet hits.
Personally, I would like to see the empowered affect of the gun differ from the selected Blitz. For ex. if you have selected the Brain Rupture Blitz, the gun will electrocute hit targets, rather than more armor piercing.
Probably allowing the player to choose between covering more bases or going all in on one direction is best however. Not sure how the devs could do that but, in my opinion, more variety is almost always more fun.