How loud are guns in Pathfinder, really?

Related question here, similar question about casting volume here.

I’m playing in a game with a gunslinger ally, and while this hasn’t come up just yet, I’m sure it will eventually. Are there any hard and fast rules for the actual DC value to hear a gunshot in Pathfinder 1e? I don’t have access to the books right now, but the SRD table for Perception lists “Sound of battle” as -10 to the DC as the closest thing, and I couldn’t find anything on the firearms page specifically.

Is there any ruling more concrete than this for the sound of gunfire, or are we in GM fiat territory?