ok if you want to desl with the inferior weapons issued to SoBs here's my two thrones on it.
Alternatively, just go with Games Workshop instead of BI/FFG (lol @ "civilian plasma guns") and give them weapons with similar profiles. That would also preclude having to find a justification for even Inquisitors apparently also getting crap ammo.
Yeah, the different stats for Astartes vs. 'civilian' bolters exists only in WH40KRP and no where in the fluff, so no extra justification should be needed- just give everyone the same kind of bolters, full stop.
But I still recommend going with an Inquisitor instead of a SoB.