Gunfire audible range

[Caradhras]

Ok - I realise this is like 'how long is a piece of string' but maybe I can narrow the parameters somewhat.

If a few short bursts of M-16 fire occured in a relatively quiet rural area - I was wondering how far the sound would travel eg neighbouring farms/towns.

Obviously applying modifiers for other aural activities - but as a general rule (because I have no idea at all!) with no real interference, how far would gunfire (using M-16 as a base) carry to be easy to hear.....