It depends on your definition of "underreported"

When you consider how often reporting gunshots or (pretty much anything) to your local PD does any good to any stakeholder I would say they're wildly over-reported.

I wouldn't doubt that guns shots are underreported. I lived in the "hood" in Milwaukee. I noticed them at first but they were so frequent that they soon blend in with the rest of the city noise. I had a new room mate move in. His first night he work me up in a panic: "What was that?!!! It sounded like gun shots!". Me: "I'm sure it was" - and rolled over. There were shots fired nearly every night. I can only guess, but I'd have to say most of those guns were not legal. I didn't once call them in. I doubt many other people did either.

Contrast that with my time living in a quiet suburb in another state. There was some public land nearby that would attract target shooters. Some neighbors were up in arms (no pun...) over it. While speaking with one neighbor she was worried about the "automatic" gunfire she could hear nearby and she was sure they were not out far enough from the neighborhood. She was right about one thing - it was in the neighborhood, but it was workers nailing sheeting on walls for a new house. They had multiple nail guns running as fast as they could. Yes - she had reported gun shots, many times. I'm sure some of them really were, but they were all very likely to be legal.