Airsoft or Airgun (or Other Projectile Throwing Device) Chronograph Using a Microphone

My first Instructable, constructive criticism welcome.

If you have an airsoft gun or any other projectile firing device that you need or want to chronograph the options are pretty limited. You can find some one else who has a chronograph, and ask or in some cases beg to use it, or you can pay out for one. I couldn't afford to buy a chronograph and don't know anyone with one so I spent a little time researching how to make one. They are not simple enough for little old me to build, so when I heard someone saying they were using an audio program to time things I thought "Why can't this apply to my needs?"
A short amount of thinking later and this is what I came up with.
(I don't think this really needs many images but if you need anything clarified I will add some.)

You will need:
1. An airsoft gun / airgun / bow / etc.
2. A weighing scales or projectiles of known mass
3. A computer with audio capture card ( one with a microphone input to be precise)*
4. A microphone
5. A measuring tape and something to mark the ground with ( chalk works great )
6. An audio capture program ( Audacity is open source, free and worked well for this application. It can be downloaded here http://audacity.sourceforge.net/)
7. A target that will cause a sound when struck and a backstop
8. As always with projectiles, A SAFE AREA TO TEST THE DEVICE AND AN APPROPRIATE BACKSTOP. ACCIDENTS HAPPEN AND SO PRECAUTIONS SHOULD ALWAYS BE TAKEN TO PREVENT INJURY.

• A device that captures audio with a high enough bit rate, preferably in .wav format could be substituted for the computer, if you can't put the computer in place, or don't have a capture card. I haven't tried this but I don't see why it couldn't work. I know there are .mp3 players out there with microphones and recording software, and usb ports are on all, if not most computers these days.

If you already have all this stuff like I did, then this can be an attractive alternative to paying out for a chronograph.

Got everything you see on this list? Good! On to step one.

If you have your safe area picked out, and ensured that nobody is going to cross your firing line, it's time to set up.

1) Get your measuring tape and measure a distance of five metres, marking with the chalk or whatever you decided to use the starting point, 2.5 metres and the 5 metre mark.
The starting point is going to be where the end of your barrel, arrow etc is placed to fire from.
The 2.5 metre mark is where the microphone is going to be placed. This helps eliminate some of the travel time error of the sound, which can be large enough to cause an error, depending on how much of an echo the area has. The 5 metre mark is where the target is going to be placed.

2) Place the microphone at the 2.5 metre mark and position it so it has no bias towards the shooter or target, i.e. it should be placed at right angles to both. Place the target at the 5 metre mark. The front of the target should be facing the shooters position and be as upright as possible. If it is tilted it will change the shooting distance. An appropriate backstop should be used ( a cardboard box probably won't stop a .177 pellet from an air rifle, I'm not sure). Load up the airsoft gun, airgun etc. and cock it if possible before the start of the test. The end of the barrel (again, etc.) of the (whatever) should be placed at the starting point or "0 metre" mark.

1) Start recording using your recording equipment and fire a number of shots (one is fine, but a few results can be averaged to better find an accurate number) at the target. I used Audacity because it was free and allows you to view the recording's waveform, which is what we want here.

2) When you are done shooting, stop the recording and save it to ensure your work isn't lost. Now is when we see how fast the projectile went.

3) Using Audacity, find the point(s) of the recording where you fired, and the target was struck. Using the zoom tool, zoom in on this point. It will be visible on the waveform where the shot was fired and the target was struck. This is the trickiest part of the entire process.

On the wave where the gun was fired (I haven't tested a bow yet), it can be hard to find the point when the shot actually left the barrel. This is due to the fact that the gun tends to vibrate before and after the BB or pellet has left the barrel.
I deduced that there would be an inconsistency in the wave and that my gun was louder after the shot had left the barrel, as it was louder when dry fired.

4) Read the time on the graph when the shot left the barrel and the shot struck the target and subtract the former from the latter. This is the travel time of the shot. Weigh the projectile if necessary. (For BBs this needs to be done in batches, unless you have a sensitive enough scales. See step 4 )

We now know the travel time, the distance and the mass, which means we can find the speed (velocity) and the energy of the projectile.

Speed = distance divided by time
Kinetic energy = 1/2 (Mass) multiplied by (Velocity }squared{) giving a result in Joules
Distance = 5 metres Time = x seconds Mass = y Kgs

Speed (velocity) = 5/x Giving a result in Metres per second
Squared means multiply the number by itself

**Example**
My Airsoft rifle

Time = 0.05755 seconds
Speed = 5 divided by 0.05755 = 86.88 metres per second
Mass of projectile = 0.25 grams = 0.00025 kilograms

Energy of my rifle = 1/2 (0.00025) (86.88 squared) = 0.9435 Joules

Because I live in Ireland where the legal upper limit on a gun like this is 1 Joule before it is classified a firearm and needs a license, I was happy with this result, and it verified the store's energy claim.
ONE LAST IMPORTANT THING

This test gives the energy at 5 metres distance NOT the muzzle energy or velocity. There is some error in the result but it is not much, however if you are trying to adhere closely to the upper legal limit etc. in your region this test may be too inaccurate for your purposes.

If you have ever had a bunch of BBs sitting around and you need to find out their weight but don't have a sensitive enough scales to weigh them individually or in small groups then this method can be of some use.

Take an even number of the BBs, about a hundred or so. Yes you will need to count them, but it's better than using BBs that your hop-up is not zeroed for, and missing by a mile when you shoot.

Zero your scales with a bowl on top of it (anything that can hold all your BBs)
Dump all your BBs in and read off the weight.
Divide this weight by the number of BBs and you have the weight of your individual BBs.

**(and then do it for a different number, try to stick to 10's and 5's at the end of this number. I made this mistake and got an inaccurate weight for my BBs because my scales only reads down to the 1 gram level and the difference between 0.20 and 0.25 can skew results badly)**

This can also be used to determine an approximate number of BBs in a bottle if you know the weight of them.
Empty the BBs into a different container while you weigh the bottle they are in. Put all the BBs back in the bottle and weigh it again. Subtract the weight of the bottle from this number.
Divide this new number by the weight of the individual BBs (0.20, 0.25 gram etc).
This gives an approximate number of remaining shots for the cash conscious airsofter!
(I know BBs are cheap, but that doesn't mean you should waste 'em, and I like knowing how many I have)