You get the first Bolter by helping the Duergar guy to fix his design (requires high intelligence), and you get the next one by killing him and picking it up from his body. I can't remember which one is the Doom Bolter and which one is the Hell Bolter. But they are both in normal mode.
There is a Hell Bolter and a Doom Bolter. One is the one he gives you for helping him and the other is the one you get for killing him. You can get both.
