Originally Posted by MontanaRifleman
The first question is... is your chrony accurate? If you think so, how do you know? Most chrony owners can not answer those questions. Even if you knew for certain that your chrony was accurate, a low ES does not necessarily make an accurate load. Correct, but a low ES reduces vertical stringing at long range and if I get low ES and good accuracy at 300 yds, I don't have to be concerned about vertical stringing at 1000 yds. I have great odds that load will be good to go at 1000yds.
The farther down range you can shoot for results, the better. I look mostly at verticle dispersion when evaluating a load.
I know of no way to absolutely prove a chrony is spitting out exactly correct velocity. This is somewhat akin to the girlfriend asking for proof that you love her. If you can't see it, it's tough to prove it. Calculating the average of two recorded velocities by shooting over two separate chronographs that have been demonstrated to operate reliably would be a step towards establishing greater assurance of an accurate, true MV. Adding a third quality chronograph would add even greater confidence. If I fired over 10 separate chronographs, tossed out the lowest and the highest velocities, and took the mean of the velocities from the remaining 8 chronographs? Now I think we'd be getting pretty darn close to the true velocity. But who wants to buy, operate, and maintain 10 chronographs? Sooooooooooooooo....
Here's how I go about improving the accuracy, credibility, and confidence of my chrony data. Using two chronys in tandem is the first step toward more credible chrony data, as stated in my earlier post in this thread, and as I have in other posts/threads in the past. So I run two chronys in tandem. Two chronys is the bare minimum required in order to be able to establish that the instruments are spitting out credible numbers. It would appear that at least one of your two chronographs isn't performing reliably, if I understood your earlier post correctly.
With the two chronys recording each shot, the next step is demonstrating that the delta between the differences in recorded velocities from the two chronographs is in the single digit range - the lower the better. By this I don't mean that the difference between the velocities reported on one chronograph for each shot string is 9 fps or less. A delta of 5 fps or less for the differences of the recorded velocities for 4 separate shots means that if the one chrony reads 3005, 3010, 3017, and 3008 fps then the second chrony reads 3005 to 3010, 3010 to 3015, 3017 to 3022, and 3008 to 3013 fps for those 4 shots, respectively. The ES from the first chrony is 12 fps, however the delta between the differences in velocities recorded by the chronys is 5 fps or less. After establishing the 'normal' delta for the chronys over repetitive shootings, we have established a means of identifying bad velocity data. If I fired a 5th shot and my chronys recorded velocities of 2981 and 3012 fps, this 31 fps difference is an obvious red flag, because I normally obtain a delta of 5 or less. Clearly, one or both chronographs hiccuped on this fifth shot and spit out a bad velocity. The 2981 fps is likely the bogus one because it's so far outside the mean of the first four shots, while the second reading is right in line with the first four shots. With consistently repetitive single digit deltas (with my chronys, generally less than 5 fps), I have a method to identify faulty velocity data from any individual shot fired. I discard these bad recorded velocities. I don't include them in any data sets when determining MV, ES, or SD. Last weekend I fired 8 shots over my chronographs and the delta was within 4 fps for that 8-shot string.
At this point in the process, I've demonstrated my chronys are operating reliably. I also obtain two readings for each shot. Each of these two measures improve the credibility of my recorded velocities compared to operating with a single chronograph.
Next step is to work up loads while recording velocity data. Loads with high ES get eliminated even if they produce good accuracy. Loads with low ES and good accuracy get further attention. I develop loads by shooting at 300 yds. If I get good accuracy and repeatable low ES at 300 yds, that load is generally going to be a winner at farther ranges.
After I've honed in on the load I intend to use, I shoot for groups at 1000 yds, recording group size and drops. If the load's still a winner at 1000 yds, then with the known 1000 yd POI, I set up my chronographs at 1000 yds and obtain 1000 yd velocity.
Can I prove my MVs or 1000 yd recorded velocities are accurate? No, not with absolute certainty.
With the MV, the 1000 yd velocity, and known atmospheric conditions, I can calculate an accurate BC for my bullet. If the chronys read high at the muzzle, they'll read high at 1000 yds also. If they record velocity slightly lower than the actual (true) velocity at the muzzle, I fully expect the 1000 yd recorded velocities will be equally low. The difference in the velocities at the two ranges is what's important for BC determination. If my recorded velocities are 20 fps high at the muzzle and at 1000 yds, or 25 fps low at the muzzle and at 1000 yds, I'll still get a very accurate BC detemination from the velocity data for my bullet. An important (to me) advantage to collecting the 1000 yd velocities is that the affect of any error in the measured velocities or the measured distance is diminished in the calculation of BC. There is such a great loss of bullet velocity that an error in recorded velocity of 20 fps has relatively little affect on the accuracy of the BC calculated over that large velocity loss.
I also use the chrono'd MV, 1000 yd velocity, and the 1000 yd drops to calibrate and verify my ballistic program. If I'm using Berger VLDs (I do for long range hunting), then I compare their published G7 BC with the G7 BC I calculate using my chrono'd MV and 1000 yd velocity. If they're close, that helps improve the confidence in the accuracy of my chronographs. I'll use the Berger BC as well as my calculated BC along with my measured MV, and see if my ballistic program's predicted 1000 yd velocity matches my chrono'd 1000 yd velocity. If it does, I have further verification that my chronographs recorded pretty accurate velocities. If the predicted 1000 yd velocity is off a bit from my chrono'd velocity, I'll tweak the BC until my program predicts my 1000 yd chronographed velocity. After the BC is tweaked (if necessary) so that predicted 1000 yd velocities match my recorded velocities, I then check to see if the predicted 1000 yd drops match my measured 1000 yd drops. If they do, I have further confirmation that my chronographs recorded reasonably accurate velocity. After these steps, I've verified my ballistic program is properly calibrated to accurately predict down-range velocities and drops.
This whole process provides confidence that my ballistic program will now closely predict the proper dope for LR shots under variable environmental conditions and sloped shots. All this in the effort to avoid becoming a victim of the garbage in - garbage out syndrome.
Nothing to it.
My question for you is how do you know you're inputing correct MV into your ballistics program without a chronograph. Do you base your MV on drops? If your muzzle velocity or BC is in error, you can develop a drop chart for one set of environmental conditions and get away with it - maybe. But if you then relocate to a site with vastly different station pressure and temperature, your drop chart will no longer be accurate, and predicted dope from a ballistics program will also be off the mark.