FYI, there is a link in a thread somewhere in this forum that tells you what the requirement is (I'm not going to say WHICH, so have fun with that), but the only real info you need is where the Dragonslayer comes from. After that, it's not hard to at least make some educated guesses. I haven't actually picked one up myself, but I can narrow it down enough to know what sorts of things I need to do/have done.
Dang, you were packing some nice numbers of mods: B3 for the BFG and P3 for the rifle is pretty awesome. Some people don't like to overpower the rifle, though, because of potential knockback. I gotta say, I'm often disappointed by Hell's Armory...never seems like it gives you a unique when you need one.
So, now that you have a victory of each kind in your pocket, feel like making your way to HNTR? It's the difficulty of angels (warning: some angels may cause sweating, nausea, headaches, diarrhea, and bleeding out the eye sockets)!