IMO, On the rifle, i would add a short nozzle where the big honkin barrel was, to give it a sort of E-11 feel to it. Whatever looks good.
I would also recommend doing away with the barretts native magazine, and covering the open space with a panel, and putting different greeblies on it, if possible. The native mag that is on their now, and the big chunky greeblie, conflict in their visual indications as to what the blaster is. is it a slug thrower ? is it a laser gun ? is it a gauss rifle ?
imo, giving the mag area the 'DL-44' treatment, covering the mag slot, giving it greeblies that suggest that part of the gun as an electronics box, would go along way to making it look Swarzy.
the pistol ? fits right in as a hold-out blaster, imo. The only thing you would need to do, if you wanted to, would be to add more silver weathering, and maybe rub the handle and 'foregrip' sections of the pistol with some deeper browns, to give it some color contrast to all that black.