Honestly, I've only seen a few quests for guns before, and the difficulty and score required to get said guns seemed to be related to the power of said gun.
For example, it was not hard for Leon Adalbert to get a flintlock. He basically participated in a gun-running thread, picked one up, and used it for a bit. It was not an incredibly powerful gun, and it took a while to load each shot. Therefore, it was easier to procure.
However, in the same thread, another character grabbed a semi-automatic handgun and tried to get that as a spoil. No go. In fact, she was allowed the gun, but it was broken and she needed to go through another quest to find someone who could forge the correct part to fix it. So, really, she was allowed the powerful handgun, but couldn't use it until she completed another thread to get it.