The formula I posted is a very simplistic one at best.
For 150dB it would be 30dB at about 11.3 km taking into account no environmental effects or terrain / obstacles to absorb the sound.
Flat plain, gun fired straight at you and sound spreads out evenly, but of course it does not.
1/6*4/3*3.14=0.697. 0.697*X^3=Y{m3} X=1 when dB=150.
11.3km you'd have 1,005,699,209,000m3 which is 120dB difference, so the sound would be around 30dB, just above a whisper.
Source :
http://www.thehighroad.org/archive/i.../t-226357.html