Robert Riggs was a gay man. Being gay in American during Riggs's time was something not to flaunt in public.

Throughout Riggs oeuvre, one sees continuous references to strong
hunky manly men and vulnerable boys. It is a core element of is
iconography and is described in a powerful graphic style.

In “Prospector firing Rifle” Riggs is clearly making a sexual
statement about the moment of climax. The long, straight rifle explodes
and is captured at peak moment of climax. Meanwhile, the two sacks of
gold clearly resemble the other part of the male anatomy. Riggs was a
thoughtful and meticulous visual thinker and nothing in any of Riggs’s
compositions were haphazardly placed. In tribute to the artist’s
suppressed feelings, I have given this work a second title “Happiness is
a Warm Gun”. It’s my believe that Riggs saw this work as more than an
editorial assignment for a national magazine. This was an opportunity
for Riggs to get pleasure in voicing his feelings, even if it was
symbolically.
"Riggs openly identified as gay to his friends in the
Philadelphia art community, but did not disclose his orientation to his
employers." - Woodmere Art Museum