If you want to do some digging, there was a very active thread early in the DW Forums debating the difference between Astartes and Civilian bolters (yes, Civilian !). It was mostly Lynata and I insisting that GW fluff makes no differentiation between the power level of bolters (only overall quality), while several fanboys insisted that SM bolters be stronger than SoB bolters because, basically, SoBs are g-g-g-g-girls- ew! At least that's how I remember it...

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Back on topic: while I can understand your desire to spin a super-detailed background for your character, Prof T, keep in mind that it will factor into your games only as much as your GM allows it to influence events. If your GM just wants to run a hyper-violent 'shoot-'em-up' campaign (and I suspect that about half of DW GMs fall into that category), then you are setting yourself up for a frustrating gaming experience as your detailed background goes untapped in favor of Action-Action-Action...

I can't remember where I read it, or even now thinking about it, if it exists outside my head, but somewhere it states that "Civilian" or human Boltguns are .50 Cal and Marine equivalents are .75 Cal - I'd assume the Table Top would class them all as equal power due to simplicity of game mechanics.

I am absolutely certain it's in print somewhere in an old edition.