That crossed my mind as well. My initial reaction was he was trying to impress someone with the gun. Perhaps an online "love" interest??? I use the word word "love" very loosely. Heinrich sounds very much like a loner to me. I can't imagine him having any personal connections other than online. JMO.
That was my initial feeling as well - that it would be a showing off type of thing to do. Which obviously leads to the question of who. And if there was someone he wanted to brag to or show off to, what else might he have told them? Could've even been someone like-minded from that online group he was a "guest" of, that he printed some of those pictures from.
My next question would be, what does a person do in order to become a "guest" of one of those groups? You'd think they would have some sort of screening process, questions asked, or some kind of exchange of information. I would imagine he would have had to connect in some way and share his proclivities with at least one other person in order to locate and gain access to such a group. Ugh.