[quote=@Andreyich]
Commissars are still Astra Militarum. They can't shoot me without getting into a lot of trouble unless I done something reeeaaaal bad.
[/quote]

Actually, they're not. Commissars are formally a part of the Militarum Tempestus as part of the Departmento Munitorum's Commissariat and are empowered to execute [i]anyone[/i] that gets in the way of proper Imperial discipline, with the only exceptions I can think of being the Adeptus Astartes and the Holy Inquisition... for obvious reasons.