My blind, intelligent species has advanced technologically to the stage of space exploration through the usage of FTL travel. The reason for their blindness is evolving underground, after the surface had been rendered uninhabitable by a GRB. The environment they were in was one of massive cave systems, which had an ecosystem capable of supporting life.
Because of this, this species relies on hearing for their primary sense, using highly sensitive "hairy" antenne on their heads to pick up sound, as well as using echolocation. They have one primary pair of arms located on their bodies, and a secondary pair of arms on their backs, which they use for emotion and cleaning their antennae and proboscis mouths. This species also has 4 legs, and an exoskeleton. These secondary limbs are highly mobile, but smaller/weaker than the primary limbs, but still able to be used for manipulation of objects, with 3 fingers on each hand.
I want to give them a unique firearm, different from humans, but I am having trouble imagining what that would look like. In combat, what would their equivalent of sights be? How could a firearm make use of the secondary limbs (which can reach across to their front). I am thinking of something similar to the bren gun, where the magazine is located at the top to make reloading easier.