Check out the links such as this ==>
http://www.sengpielaudio.com/calculator-speedsound.htm which gives you the speed of sound against a particular temperature. I drew a graph up of the temperature range in my area (plus and minus some degrees) and learnt a lot about why some bullet loads seem louder.
The other thing to watch out for is noise reflection off solid objects such as trees. Always make this seem noisier.
Finally, get someone else to shot your loads and walk around in back and sides (safely of course!!) at different distances from the firer and see what your perception of the bullet flight sound is like then.
Perception is subjective. Behind the rifle there is too much else happening to really be able to tell what noise the bullet is making at times.
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