Brian, what formula would you use to work backwards if you know the speed of a bullet at 1000 yards? Reason for this is electronic targets give you this info. Is that speed accurate and have you tested any of these units?
Bob
Bob
simple math to work backwards
- Thread starter Bob Sebold
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Bryan Litz Ballistics
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I don't have a good idea of how accurate those targets are at measuring velocity. I would be surprised if they're accurate enough to determine BC; they would need to be +/- 5 fps or better in absolute accuracy which is harder to do than you might think. It will be interesting to see how spclark's test with the Oehler compares.
Another thing to remember when measuring BC's; it's not just the accuracy of the downrange velocity measurement, but you also need to know the exact range. You cannot assume it's exactly 1000 yards from the muzzle to the target velocity measurement just because you're on a '1000 yard' range. You also have to know the atmospherics, and measure your muzzle velocity so you know how much velocity was lost. It would be a mistake to assume the MV of each shot was the average you measured last time, you need to measure at the time of the test.
After thinking about all that, once you have solid numbers, you can use a number of calculators. Just plug in your conditions and itterate on BC input until the calculated downrange averages matches what you measured, then that's the right BC. One caveat; the BC you reach using this method will be valid only for the speed range over which the bullet flew. This will be a highly important thing for G1 BC's since they're very velocity sensitive, but if you calculate a G7 BC based on downrange measurements, it will be more accurate over a wider range of speeds.
Take care,
-Bryan
Another thing to remember when measuring BC's; it's not just the accuracy of the downrange velocity measurement, but you also need to know the exact range. You cannot assume it's exactly 1000 yards from the muzzle to the target velocity measurement just because you're on a '1000 yard' range. You also have to know the atmospherics, and measure your muzzle velocity so you know how much velocity was lost. It would be a mistake to assume the MV of each shot was the average you measured last time, you need to measure at the time of the test.
After thinking about all that, once you have solid numbers, you can use a number of calculators. Just plug in your conditions and itterate on BC input until the calculated downrange averages matches what you measured, then that's the right BC. One caveat; the BC you reach using this method will be valid only for the speed range over which the bullet flew. This will be a highly important thing for G1 BC's since they're very velocity sensitive, but if you calculate a G7 BC based on downrange measurements, it will be more accurate over a wider range of speeds.
Take care,
-Bryan
I am the manufacturer of the referenced hardware (Silver Mountain Targets).
The measurement of the bullet speed at the target is made by a pair of microphones mounted in the lower left corner of the target, with one microphone directly in front of the other by about 8 inches (200mm). The time of the bullet's shockwave arriving at each mic is measured (with a sub-microsecond timer) and from this the speed of the bullet is calculated. Nominally the two microphones are supposed to be "straight", pointing directly at the shooter, though in the real world there are invariably mounting errors that result in slightly crooked mic alignment.
The at-the-target velocity reading is *NOT* an "instrument-grade" measurement. It is a surprisingly precise (repeatable) measurement, but as far as absolute calibrated accuracy goes, it is extremely sensitive to very small errors of misalignment (as in, a few mm of misalignment will give you about a 1% error, and in practice under most ordinary microphone mounting situations it is quite difficult to avoid small errors like this). It is quite good for determining how consistent your bullet speed is upon arrival at the target, and if you do careful work comparing an instrument-grade chrono at the muzzle to our e-target's at-the-target acoustical chronograph you will usually find extremely good correlation (unless there is something genuinely weird and interesting happening to your bullets enroute to the target).
The speed measured at the target by our SMT system is specifically not (nearly!) accurate enough to use to calculate bullet BC based on speed-at-the-muzzle and speed-at-the-target measurements. Measuring BCs in this manner is a difficult task, surprisingly sensitive to very small measurement errors, requiring extremely good absolutely accuracy in both speed measurements as well as (as Bryan says) the exact distances, and also reasonably good measurements of air density.
The measurement of the bullet speed at the target is made by a pair of microphones mounted in the lower left corner of the target, with one microphone directly in front of the other by about 8 inches (200mm). The time of the bullet's shockwave arriving at each mic is measured (with a sub-microsecond timer) and from this the speed of the bullet is calculated. Nominally the two microphones are supposed to be "straight", pointing directly at the shooter, though in the real world there are invariably mounting errors that result in slightly crooked mic alignment.
The at-the-target velocity reading is *NOT* an "instrument-grade" measurement. It is a surprisingly precise (repeatable) measurement, but as far as absolute calibrated accuracy goes, it is extremely sensitive to very small errors of misalignment (as in, a few mm of misalignment will give you about a 1% error, and in practice under most ordinary microphone mounting situations it is quite difficult to avoid small errors like this). It is quite good for determining how consistent your bullet speed is upon arrival at the target, and if you do careful work comparing an instrument-grade chrono at the muzzle to our e-target's at-the-target acoustical chronograph you will usually find extremely good correlation (unless there is something genuinely weird and interesting happening to your bullets enroute to the target).
The speed measured at the target by our SMT system is specifically not (nearly!) accurate enough to use to calculate bullet BC based on speed-at-the-muzzle and speed-at-the-target measurements. Measuring BCs in this manner is a difficult task, surprisingly sensitive to very small measurement errors, requiring extremely good absolutely accuracy in both speed measurements as well as (as Bryan says) the exact distances, and also reasonably good measurements of air density.
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