Ok this may have been posted here before, but i case it hasnt it makes for interesting reading...
So, what i did was took a laptop and a microphone into the pits below the target whilst my friend shot from 1000yds away with his .338LM using 250gr scenars.
I recorded the "soundtrack" using "audacity" and then once id collected the data, i used the spectogram waveform to isolate the time difference of the sonic crack of the bullet flying overhead from the faint muzzle report of his rifle a short time later.
Using the Time of Flight in a ballisitcs calculator, and the time it takes sound to travel 1000yds @ 26deg C, i could accurately predict his muzzle velocities to less than 1% error assuming we had an accurate G7 BC (not velocity dependant, i used a G7 of .332 for the 250scenar) and good environmental conditions measured from an accurate instrument (i used the pressure and temp readings from the Leica CRF1600). I recorded a string of 5 shots, resolved the velocities and even gave him the extreme spread for his load that was near dead on!
So, Using similar ideology in conjunction with an accurate chrony at the muzzle, we could reverse calculate a G7 BC in the same way. Beleive it or not, it is surprisingly accurate and you can clearly record time differences of .0005 seconds resolution. For those that cant be bothered reading the article, every 0.0005sec time difference in the calculations gives about a 1fps difference in predicted velocity.
Heres an article that explains the idea... - http://arxiv.org/ftp/physics/papers/0601/0601102.pdf
We also tried it @ 50yds and It works damn good is all i can say... we had good agreement with the chrony in everything we shot, ie less than 1% error. Just thought you might be interested...
So, what i did was took a laptop and a microphone into the pits below the target whilst my friend shot from 1000yds away with his .338LM using 250gr scenars.
I recorded the "soundtrack" using "audacity" and then once id collected the data, i used the spectogram waveform to isolate the time difference of the sonic crack of the bullet flying overhead from the faint muzzle report of his rifle a short time later.
Using the Time of Flight in a ballisitcs calculator, and the time it takes sound to travel 1000yds @ 26deg C, i could accurately predict his muzzle velocities to less than 1% error assuming we had an accurate G7 BC (not velocity dependant, i used a G7 of .332 for the 250scenar) and good environmental conditions measured from an accurate instrument (i used the pressure and temp readings from the Leica CRF1600). I recorded a string of 5 shots, resolved the velocities and even gave him the extreme spread for his load that was near dead on!
So, Using similar ideology in conjunction with an accurate chrony at the muzzle, we could reverse calculate a G7 BC in the same way. Beleive it or not, it is surprisingly accurate and you can clearly record time differences of .0005 seconds resolution. For those that cant be bothered reading the article, every 0.0005sec time difference in the calculations gives about a 1fps difference in predicted velocity.
Heres an article that explains the idea... - http://arxiv.org/ftp/physics/papers/0601/0601102.pdf
We also tried it @ 50yds and It works damn good is all i can say... we had good agreement with the chrony in everything we shot, ie less than 1% error. Just thought you might be interested...