I want to offer my genuine opinion here, but it is nothing more than that; the gun looks 'fake' to me, like a plastic toy almost. It's possible it could be an airsoft gun or something along those lines. There are some crazy replica airsoft guns out there that even use full on metal and wood to fully try and replicate the weight and feel of the guns. That said, there is a long running (from way back, original black and white cartoons even included it) joke here about guns and proms and weddings and such. Shotgun wedding comes to mind. So this plays into that without having to be necessarily racist.
Beyond that, as I could be completely wrong (again just my opinion there and as follows) the tone of the picture, as it were, seems a bit off to me as well.
This looks like a prom pic, or something along those lines. Homecoming? Uh... what was that other one... Sadie Hawkings? Something like that? To me this implies that they've probably already been involved / dating, so the dad is likely familiar with the kid and the kid him.
Also, and again, going on a single picture (which I see a lot of folks in the comments doing, but in a different direction than I am) that I see before me, the kids body language (either of them honestly) doesn't look in the least bit put off or scared.
I guess what I'm saying is, to me this looks like a joke picture between friends and family, and that the kid everyone is worried about was 100% in on and understood, and that it probably is a simple as that.
I love this sub (as an American) for the same reasons you do, it's entertaining to read / see the really dumb extreme shit out this way.
But I also get dismayed at thinking that (in the hypothetical of course, as I have no evidence nor have conducted any studies) younger or more impressionable folks from your respective and wonderful countries might start seeing that as all of us here.
That's why I just wanted to offer my opinion on the picture, not to excuse it or try to say it IS what I say it is, but just to try and give another possible side of the coin.