For non gun users - I recommend watching the heist/ambush scene in Heat at a painful volume. This is a reasonable simulation of the loudness.

Exactly. Shooting a simple tiny slow 9mm (classic pistol cartridge, used ie by cops/military everywhere) has in indoor range such a blast you feel the3 shockwave in your guts. Even with hearing protection its a very intense experience that you need to get used to. You simply don't get anything similar elsewhere, unless some big fireworks blast very very close to you.

Not using ear protection constantly around guns will degrade your hearing significantly and permanently.

Guns are very loud close up, but even a moderate distance and especially in an urban setting where sounds bounce all over the place and get partly diverted, they rapidly seem a lot less noisy. I live in a place that just a few years ago had regular gun battles just blocks away from my home and while you learned to rapidly identify the specific sound from things like firecrackers, it wasn't especially loud unless you heard it from very close by.

This isn’t about firing guns. It is about hearing a gun fired some distance away from you in an open-air urban setting. Has that ever happened to you? I have unfortunately been in that situation multiple times due to the neighborhood I used to live in. It really does sound like a clap, not a bang, but much louder. But not deafening loud as Hollywood would have you believe (unless it goes off right next to you).