By what I know most backpacks in the Warhammer 40k universe depend on the space marine. Which in this case would have to be a bullet storage unit (Possible power supply) since the unit in question is a heavy weapons. If he was wielding a chainsword he'd have a jetpack, and if he had a normal bolter it would be a basic pack. But one can't judge to harshly.
I know I looked it up. Funny thing is the sisters of Battle are a part of the "church" not the military. and they aren't even part of the inquisition, but work closely with them. plus... HERESY YOU SAY?!