That's not what Sinon says?
Going back and looking, she says something like "A Black Star 54... What? That gun... What is that gun doing here now?"
The odds that DG would be killing people with the same model from her trauma are ridiculously low. Probably nobody even uses that gun in the game because there is just better stuff.

I feel really bad for random eyepatch dude that ambushes Kirito and Sinon. He empties a whole clip, and is like "WTF!?", reloads, empties another clip, then gets just totally owned.
Sinon's rifle is really ridiculous.