Poor but not pop can poor chronie

[pizzainthemorning]

First, this is not a proposed method to determine the exact fps of a given gun, for that use a real chronograph. However, for the vast majority of us that don't have a chronograph and the few of us that have a lot of time on our hands, maybe this will work.

To the airsoft physicists:

It's a pretty simple idea, I'm just looking for feedback and ways to improve accuracy (and hopefully I've done it right...).

The Basic Idea:
Essentially I recorded the sound of my gun shooting a target at a measured distance. My recording contained the time the gun was shot, and the time of impact (as it could be heard). I then had a distance and time it took to get there, so I can calculate average velocity.

Achieving Accuracy:
I did my tests at 20 feet because I wanted to obtain a minimum drop in velocity. This means that the times were down to the thousandth of a second (so you need a decent recording program, I used Cool Edit Pro). To achieve the greatest accuracy, I had to
1. Correct for the speed of sound.
2. Estimate the exact time of firing.
3. Attempt to convert average velocity to muzzle velocity

Correcting for the Speed of Sound:
The microphone was positioned next to the gun, so I considered the difference here to be negligible. However, the target was 20 feet away, and correcting for sound meant that the bb hit my target approximately 0.018 seconds before it was actually recorded, so 0.018 has to be subtracted from the total time. velocity of sound = 344 m/s + 0.6T (where T is degrees C)

Estimate the Exact Time of Firing:
Here is the hardest and most questionable part. When analyzing the sound wave, you can see two distinct, and one not so distinct pattern. First there is the obvious gear wind up, third there is the loud crack the piston makes as it slams forward, but in between there is the slightest bit of "quiet". This is the time where the gears are no longer in contact with the piston and it is actually moving forward. I reasoned that this was the beginning of the shot, the time at which the bb started to travel down the barrel.

This is where it gets a little more sketchy. We use the pop can chronie to help us estimate some better values for the microphone chronie. If your pop can chronie gives you somewhere around 300 fps say, you can use the equations:

v squared / 2d = a (where d is the distance of the barrel)
t = v/a

Note: You'll have to use m/s and meters, not feet. Also, even if your pop can chronie is off by a good 100 fps (in which case you've done something very wrong), your values here in the end will only be affected maybe 10 fps.

From this, you can achieve an estimated time it takes your bb to reach full estimated fps at the muzzle. So if we look above to the start time of our shot above, when the piston first starts to push air, add this calculated time after that, and that now becomes the starting point of your time (for me, it was about 0.007 seconds after the start of the "quiet"). Measure that time to the time of impact, correct for the speed of sound, and you have a final time of flight. v = d/t and you have an average velocity.

Attempt to Convert Average Velocity to Muzzle Velocity:

http://cybersloth.org/airsoft/trajectory/07-A-01.htm
This is an amazing site with tons of info. I pulled some data from one of the charts regarding the fps drop versus distance. It varies a little depending on your initial fps, but in general, the muzzle velocity is 11-15% greater than the average velocity (over 20 ft).

Conclusion:

I've got a stock TM Mp5 and it reads about 265 fps with these calculations, which makes sense, but I have yet to get it chronied.

Hope this is at least semi-useful, and if it isn't, well, it's still a good idea.

Alex

[The Saint]

http://www.arniesairsoft.co.uk/forum...3&st=0&start=0

Like that?

[pizzainthemorning]

Yep.... like that. I should have known it'd been done already.

[Uziguy]

Thats a good way to do it. I used to use the pop can test for all my chrono needs, and then I finally broke down and bought an Excortech for $100 shipping included from WGC and i've never looked back.

Its a great chrono that measures fps and rof. The rof option is nice considering that I only collect GBB SMG's. It was cool to see my Maruzen
MP5K shooting 27 rounds per second at 320 fps

[seabass]

That is a bad ass collectioin of guns