Bart,
I've run through the free recoil calculation before, and a 7 lb rifle firing a 155 grain .308 bullet at 2900 fps is traveling about 10 fps rearward at the moment the bullet base arrives at the muzzle. Since you saw a bigger velocity drop than that, I suspect it has something to do with rearward recoil packing the burning powder forward in the case. That could slow ignition and increase what QuickLOAD calls the Weighting Factor. Only a 10% increase in the amount of powder going forward with the bullet would take another 10 fps off the velocity.
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Originally Posted by wncchester
I KNOW my chronograph is not stable enough to depend on to count within 10-20 fps around 3,000 fps on a daily basis; in fact, the thermal drift effects of a 10 degree ambient temp change is probably going to change both the clock frequency and counter stability that much or more!
…Slightly changing the bullet's passage angle OR it's height thru the sky screens will too…
…those detector things aren't made with high value optics so they aren't all that precisely focused.
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I think your last point is the salient one: the detectors are the weak spot. Since there is only a small overall light level change from unfocused shadows that they are detecting, their sensitivity would have to be controlled to match for high location precision by that method and the bullet would have to pass over each screen at the exact same height. But I think most units just rely on the sensor's native sensitivity and the distance between the screens and the use of cylindrical lenses to minimize the error. So it's not a high precision measuring approach and I think is a reason changing light conditions affect some units badly.
RSI's Jim Ristow says the CED and Oehler did best compared to Doppler radar in German military testing of units intended for armorers. But that was before the German made KVM-21 was available.
This 2006 review claims two PVM-21 units agreed within 1.5 ft/s on a .223 round's velocity. It's the only unit I know that asks the bullet diameter before you shoot and makes an adjustment for that shadow size to avoid false triggering. It reads horizontal IR illuminated screens and has a 16 MHz clock.
The crystal clock oscillators really shouldn't be an issue. To drift 10 fps out of 3,000 fps (0.33%) over 10°F, you're talking 0.033%/°F, or 0.06%/°C, or 600 ppm/°C drift. I've been designing circuits for fifty years, and I would be challenged to design a crystal oscillator that unstable. The least expensive, lowest precision 4 MHz crystal that Digi-Key sells has 100 ppm maximum frequency drift over -40°C-+85°C, with an absolute accuracy of 30 PPM. Worst case error due to drift over that whole temperature range and combined with a center frequency at its most extreme tolerance limit should be on the order of ±0.2 ft/s from that source.
Velocity error due to angling shots across the screens is a concern with compact chronographs (1 foot screen spacing). The error will be unilaterally slow readings, and it will be proportional to one minus the cosine of the angle off the centerline axis through the screens. For 10 fps out of 3000, that angle will be arccosine of 1-0.003333, or the arccosine of 0.9967, which is about 4.7°. It's possible for someone with 1 foot screen spacing to make that kind of alignment error. If you have 4 ft spacing, like the Oehler long pipe, it's nearly impossible to settle for such poor alignment as the screens then have so much overlap they are hard to shoot through without damaging the chronograph.
Some time ago I drew up what the different degrees of error would look like in the horizontal plane to a shooter 15 feet from the midpoint between screens. It shows 1 ft, 2 ft, and 4 ft sky screen spacing, with the screens rotated off-axis with the line of sight at 2° intervals up to 10°. I put all the angle error horizontal for simplicity, but tilt on any axis affects velocity readings the same way.
A 10 fps error at 3000 fps would be between rows 3 and 4 in the illustration, with the 1 ft spacing being the left column and 4 foot spacing being the right column. You can see how someone might settle for that degree of misalignment with 1 foot spaced sky screens, but with 2 foot spacing, it would be less likely, and with 4 foot spacing, highly improbable.
