In Rogue Trader everything had bolters, but the looks of bolters (or other weapons) could have many forms, so I think of it like a type of weapon which can come in many forms rather than a single model of weapon.
So even though a Zoat, Space Marine, Ork or whatever can have a bolter, it might be the same model, but could just as well be a completely different weapon just with same type of ammo and stats.
It makes it kind of more easy to have a few weapons to chose from instead of inventing different stats for every race, faction and character class.