You have all probably heard of the chrono where you put the two pieces of paper a foot apart and recording the sound using audacity or similar. If so, then you probably tried it, just to discover how inefficient it is. With the echos, the sound from the gun, and the bb hitting other things, you just can't figure out which sound spikes are the bb hitting the paper. Using the same concept, you can get very accurate readings without having to fiddle with some IR leds.
Basically, you are getting the same readings, but without all the excess noise from the gun and everything else mentioned above. By using the male plug from a pair of ear buds, you can connect them to the computer and still record like a microphone.
Making the base:
You can really use any length you like but the longer the bb has to travel, the slower and less accurate it gets. But, the shorter the distance, the harder it is to distinguish the two spikes. I like the length between the two sensors to be exactly a foot so it is easier to distinguish the spikes for high powered rifles (you guys have sniper rifles I hope) and will not slow the bb down too much for any gun.
The base needs to have two holes for the sensors, so a cardboard box is good for the base. Cut them on opposite walls and have them somewhat level with each other. You also need to have them about the same size, or have the second wall (the one the bb will go through second) larger so there is less of a chance of missing and hitting the box.
Making the connectors:
This is easy if you know how to do it. You need an old pair of ear buds you don't need, or any pair of earphones. you will be cutting off the buds and just using the wire and plug. This may not be long enough depending on how long the wires are, so take some extra wire you have or go to radio shack or home depot and get some.
After getting the wire, you need to strip the ends to have the wires bare. Odds are, there will be some sort of thin enamel coating that you can't just scrap off. You will need to use a lighter or match and burn it off to make it bare. These will be used to connect to the sensors and to your computer.
Making the sensors:
The sensors are just some aluminum foil and paper sandwiches. Take two pieces of foil and put one on either side of the paper, and the other on the second side. You need to make sure they do not touch, so make the pieces smaller than the paper.
Then connect a wire to each piece of foil. Just tape it on, as you will need to take it off later to throw away the sensor and replace it. One sensor gets one pair of wires from an ear bud. It will not work if you take one wire from one bud, and another from the other and put them on one sensor.
Place the sensors on the base:
Tape the sensors over the holes you made previously in the box. Try to have the wires not over the holes so you can't accidentally shoot them.
Now that you have it made, you need to know how to use it. Get a program like audacity or anything you can use to record and see the sound levels of the recording. I like recording at 48000hz because my computer is not fast enough to record at 96000hz.
1) Start recording
2) shoot through the two sensors (use foam or more cardboard as a backstop)
3) Find the two spikes in sound level and zoom in until you can see it up close
4) Find how long it took for the bb to pass from sensor one to sensor two
5) Divide the distance by the time to get speed. ex: 1 (foot) / 0.002 (seconds) = 500 (fps)
Hope this helps. If I made any mistakes or if you think you can make modifications, I'm open to suggestions!