Very simple multi microphone based system with interface to PC 
parallel port.

        http://www.elecdesign.com/Articles/ArticleID/2513/2513.html

Stunningly trivial circuit diagram

        http://www.elecdesign.com/Files/29/2513/Figure_01.gif

Three microphones give you both muzzle velocity and ballistic 
coefficient.
This is so cheap to do that you could make a bank of, say, 8 and see 
if the bullet baheaved as expected ballistically over a small 
distance.
eg coning will alter what happens over time. A marginally stable 
bullet may well change it's apparent BC over a short distance.. .


____________

Optical method

        http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=3597 
.. .

Circuit

        http://www.elecdesign.com/Files/29/3597/Figure_01.gif .. .

They note:

        Optical detection of a projectile whizzing past a given point 
is not always trivial-especially for BBs and other small-caliber air 
gun pellets that may fill only a small fraction (about 1%) of the 
detector's field of view. The logarithmic optical-detection method 
employed here is borrowed from an earlier IFD ("Available-Light 
Phototachometer Simplifies Outdoor Remote Sensing," Electronic Design, 
January 25, 1999) and has a wide dynamic range compatible with 
low-contrast signals in both outdoor and indoor lighting.



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