Well i did something interesting today...

[groper]

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...

 

[flyboy]

Re: Well i did something interesting today...

That is a interesting Idea. Way to go on your efforts.

 

[BryanLitz]

Re: Well i did something interesting today...

That is very similar to how I measure BC's. The exception is that I use a microphone at the muzzle and another microphone downrange that transmits it's audio back to the firing line. The two signals are mixed and recorded. Then I look at the time of flight and measured muzzle velocity to get BC within 1%. This can be done with as many microphones as you care to set out until the transmitters begin interfering with each other.

I tried doing it just how you described but found it difficult to resolve the exact time of arrival for the muzzle blast. At that distance the sound is so muffled and spread out that I found it difficult to pinpoint the exact moment the sound arrived.

Next time you have someone in the pits, keep your laptop at the bench and give him a radio. Tell him to key the mic just before you shoot, and record the audio from your firing point. You'll record the muzzle blast at your location, as well as the crack over the radio when the bullet passes him in the pits. I think this will give you better resolution. If you accurately measure MV, this technique will allow you to back out BC's extremely accurately. Remember to account for the delay it takes the sound to travel from the muzzle to your recorder, and from the trajectory to the radio in the pits. It's about 0.001s/foot delay.

-Bryan

 

[groper]

Re: Well i did something interesting today...

yes, i was most surprised at the accuracy delivered.

Perhaps the geographical topography of our range makes it easy to record the muzzle blast? our range is very open, flat country with no hills or anything but trees and grass around for miles... The conditions were ideal, just the slightest of breeze from behind the shooter and an otherwise very quiet early morning. The muzzle raport was clearly visible on the spectograph and i simply used the first wave peak of each recorded sound as my time reference point. The microphone was zip tied to the bottom of the target so the bullet would not have passed more than 2 feet from it. I guess i should allow a couple thou next time to get things really spot on...

Bryan, If im only bothering to measure VLD type bullets and use a G7 BC calculation, would this make using a single mic accurate enough as opposed to using mulitple mics at varying distances downrange for a velocity braketed G1 BC calculation?(which seems kinda pointless/obsolete trying to calculate G1`s anyway)

 

[BryanLitz]

Re: Well i did something interesting today...

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: groper</div><div class="ubbcode-body">Bryan, If im only bothering to measure VLD type bullets and use a G7 BC calculation, would this make using a single mic accurate enough as opposed to using mulitple mics at varying distances downrange for a velocity braketed G1 BC calculation?(which seems kinda pointless/obsolete trying to calculate G1`s anyway)</div></div>

Yes, one mic is enough if your referencing your BC to an appropriate standard.

I've found in my testing with multiple mics that it's very difficult to position them accurately enough to resolve the difference between similar drag curves. If you're shooting VLD's and referencing BC's to G7, there will be little room for improvement on your results.

You mentioned something that will be important to consider using your method: wind. If there's a 5 mph tail wind, it will carry the muzzle blast to the pits about 7 fps faster than your speed of sound calculation would imply. It's also important to measure the exact distance to the target with a LRF. If it's actually 1005, or 997 yards, that will make a significant difference in your tof derived BC's. Also need to account for velocity lost from muzzle to chrono, typically 3-5 fps.

The devil's in the details.

-Bryan

 

[groper]

Re: Well i did something interesting today...

indeed, the main problem with getting things right is having perfectly accurate environmental and physical conditions in the calculations. Ideal conditions to perform the testing such as zero wind, and perfectly accurate distance measurements. Whoever built our range, seems to have done it well as all the shooting mounds from 100-1000yds i have ranged the target with both zeiss and leica LRF`s and the distances are always spot on - at least to the instruments accuracy.

Thanks for your input Bryan, i might have to do some preliminary BC calculations for those strange looking projectiles ive been emailing you about

 

[The Mechanic]

Re: Well i did something interesting today...

I had always thought of having two chronos and say a flash at the moment it passes the first set of screens and a photosensor at the next chrono that could act like a third screen 100 yards or so apart.