Well, that was in the episode, and I watched it. I was just letting people know who the shooter was and what was going on. My take away was the people with Downs Syndrome don’t need to bring guns to school.
OK ... so the show is propagating yet another leftist boogieman. With one common exception (historically correct copies of the original 1873 Colt SAA revolver), dropped guns don't "go off".
They just don't ... they're very carefully designed not to.
My takeaway from your description is that the producers of "Glee" don't know their collective arse from a hole in the ground, and are pushing a variety of forms of leftist propaganda.