When Oehlers cost a lot of money,and were the only real units available), I built a ballistic chronograph as a project. It was actually surprisingly easy. A quartz crystal oscillator,I used 10 MHz), a high speed counter, and a Z-80 microprocessor to do the math.
The formula used for computing the velocity is v = l*f/c, were v is velocity in ft/s, l is the screen spacing, f is the oscillator frequency, and c is the number of counts obtained as the bullet travels between the screens.
There are a number of sources of measurement error.
The sensors have a small delay, but as long as both are the same the delay can be ignored since the start and stop signals will be delayed the same amount.
Another source of error is the screen spacing. Since the actual point in space that trips the screen is not really known, again making the screens as close to identical as possible is important, as well as measuring the spacing very accurately. The percent error on the spacing will be reflected directly in an error in the readings. A 1 foot spacing will need to be accurate to about 0.1% or so, or less than 1/64 of an inch. A 2 foot spacing better than 1/32 inch, a 4 foot 1/16 inch, etc. For a 1 % error the accuracy is 10 times less, allowing the 1 foot spacing to be within about 0.1 in.
The next source of error is the crystal frequency. This is very small since even inexpensive oscillators are within 100 PPM,including error and temperature changes). This is 0.01%. A 4000 ft/s reading will be in error 0.4 ft/s
The math required also produces a limit to the accuracy based on the crystal frequency. Time is divided into small periods, but only whole periods can be counted. Oehler uses a 4 MHz oscillator, for a period of 250 nanoseconds. Around 2000 ft/s each count starts to change the velocity by 1 ft/s. At 3000 ft/s, each count is worth about 2 ft/s, and at 4000 ft/s each count is up to ~4 ft/s. The only way to beat this problem is to use a larger spacing or a faster oscillator,or both).
With a 4 foot screen spacing and a 10 MHz oscillator the unit can be used up to ~6100 ft/s.
Oehler provides enough detailed information to allow a selection of spacing to get whatever accuracy is desired within the 4 MHz oscillator. Shooting Chrony claims around 0.5%, and PACT has no information at all.
Any are likely to produce useful data that is accurate enough to tell what is going on. Oehler allows the most manipulation with variable screen spacing.
