Being in Montana where you hear a lot of gunfire in the fall when hunting starts, I can say the following:
Average light firearms, such as a .22.. you're only going to hear it about a half mile away...maybe a bit further (sorry, this is going to be in Standard measurements, not metric...my brain doesn't work well in metric at night). Hunting rifles can generally be heard up to about 3-4 miles away. That's hunting rifles fired in the forests and I'm standing out in the open fields. It might be the echo giving it away, but you can pretty distinctly hear the muzzle discharge too. When I'm also in the forest, and the rifle discharge is in the forest, cut that down to about a mile, maybe a bit more.
I'm about 2 miles from a rock quarry where they blast about every day. Due to the duller nature of the explosions, and the fact that they're buried munitions, I can hear them when I'm outside, but not inside. Some days I can even BARELY feel slight taps in the ground when they blast. Being a once-miner, I know how powerful those blasts are up close. I've been too close to one before and was stunned by the concussive blast of one. An exposed explosion, I would imagine, could probably be heard for much, much further.
Now I'll also mention that I KNOW humidity plays a big part in how the sound travels. I moved from a very wet area, where the sounds of things were pretty quickly stifled over very short distances, to a pretty dry area (average humidity is about 16%) where sound seems to travel quite well. So I've noticed a difference in that regard.
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