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.
Got everything you see on this list? Good! On to step one.
Step 1: Setting 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.