Chronograph numbers
- Thread starter Aguila Blanca
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yet at 27.3 gns which had a ES of 8 and a SD of 4 the group height was over .5 inches while at 28.9 gns we had a ES of 25 yet the group height was only .059. Why? My guess is shooter error and wind, the wind rules all in this game. That plus barrel harmonics. Maybe, possibly, could be. I suppose I could run the test a half dozen times to see if the results duplicated. Now if I had a 300 yard testing tunnel and a machine rest in my basement oh the testing I would do
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Yet, you don't believe that if the 28.9-grain load also had an ES of 8 and an SD of 4 that it would necessarily become bigger. More importantly, if the ES and SD numbers for the two groups were flipped, you wouldn't be jumping up and down and saying that tight ES was the key to accuracy, because, as your actual numbers show, velocity is not the dominant term influencing precision here. But trying to see these things is how confusion in our efforts arises. We have to isolate variables from one another to see their effects independently.
Incidentally, I accidentally copied the windage number for gusting 5-7 from 90° at 100 yards when I meant to describe drop. That windage effect over a 2 mph difference is what is the tiny moa. If the gun barrel were perfectly rigid, you would get a drop of over a 100 fps difference of about .17 moa. That's more like what I've calculated in the past. I patched the posts to reflect the correction. But these two numbers are further evidence you are seeing the effects of muzzle vibration in response to recoil moments and pressure distortion.
A really common thing to see is a vibration that switches between vertical and horizontal ellipses as the barrel time shifts, having minimum area (smallest CEP) in the transition between these two extremes, where it is circular. So I looked at your target and saw it seemed predominantly wide at the low charges, gaining some vertical, but also some width in the middle (a bad precision node to my way of thinking) and then back to predominantly horizontal (wide) again at the end. What I look for in that sort of situation is where the two trends crossover, as that is likely to be an area minimus for both modes.
In the plot below, you can see the vertical has a couple of apparent flat spots. One between 26.4 and 26.8 grains and the other at 28.0 and 28.2 grains. In both instances, the horizontal dispersion range crosses them over at about 26.6 grains and 28.1 grains. So I would try those two loads.
Incidentally, I accidentally copied the windage number for gusting 5-7 from 90° at 100 yards when I meant to describe drop. That windage effect over a 2 mph difference is what is the tiny moa. If the gun barrel were perfectly rigid, you would get a drop of over a 100 fps difference of about .17 moa. That's more like what I've calculated in the past. I patched the posts to reflect the correction. But these two numbers are further evidence you are seeing the effects of muzzle vibration in response to recoil moments and pressure distortion.
A really common thing to see is a vibration that switches between vertical and horizontal ellipses as the barrel time shifts, having minimum area (smallest CEP) in the transition between these two extremes, where it is circular. So I looked at your target and saw it seemed predominantly wide at the low charges, gaining some vertical, but also some width in the middle (a bad precision node to my way of thinking) and then back to predominantly horizontal (wide) again at the end. What I look for in that sort of situation is where the two trends crossover, as that is likely to be an area minimus for both modes.
In the plot below, you can see the vertical has a couple of apparent flat spots. One between 26.4 and 26.8 grains and the other at 28.0 and 28.2 grains. In both instances, the horizontal dispersion range crosses them over at about 26.6 grains and 28.1 grains. So I would try those two loads.
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I am not concerned with the horizontals during the initial testing. The G1 BC of that bullet is .255 and according to JBM calculator a full force 5 MPH wind would move the bullet .5 inches. Now I shoot over flags a lot so I have seen updrafts, downdrafts, swirls etc on the range.
Just out of curiosity Nick did you plot the ES in relation to the vertical, I did not myself because just at a glance there is no correlation. I ended up loading at 28.1 gns. It is just a practice load for 300 yards but is remarkably accurate, (relatively) cheap to load, and conserves my Varget
Just out of curiosity Nick did you plot the ES in relation to the vertical, I did not myself because just at a glance there is no correlation. I ended up loading at 28.1 gns. It is just a practice load for 300 yards but is remarkably accurate, (relatively) cheap to load, and conserves my Varget
Yah, I messed up. I used Didion's formula for deflection of the difference between 5 and 7 fps then forgot to convert feet to inches. I also was going too fast and didn't note your wind was shifting from side to straight ahead.
That blue line is the ES in vertical. The wind probably explains a good portion of the horizontal in the middle of the plot. But what I am seeing that mainly caught my eye is the shift in vertical from middle to low to high and back down again, and that is suggestive of the muzzle vibration shifting in and out of a vertical mode. The Horizontal, either side of the center, appeared to be running opposite the vertical, which is what suggested you may have the vibration modes trading off over time. But separating the horizontal vibration from the wind is tough. You could try the two charge weights I suggested where the lines cross. But if the vibration is all vertical and the wind is doing all the horizontal, then you want to go back to 25.8 and 28.6 when the wind isn't blowing.
That blue line is the ES in vertical. The wind probably explains a good portion of the horizontal in the middle of the plot. But what I am seeing that mainly caught my eye is the shift in vertical from middle to low to high and back down again, and that is suggestive of the muzzle vibration shifting in and out of a vertical mode. The Horizontal, either side of the center, appeared to be running opposite the vertical, which is what suggested you may have the vibration modes trading off over time. But separating the horizontal vibration from the wind is tough. You could try the two charge weights I suggested where the lines cross. But if the vibration is all vertical and the wind is doing all the horizontal, then you want to go back to 25.8 and 28.6 when the wind isn't blowing.
That begs the question, how do you define "minor". If you accept that all the groups in your target are within the range of statistically random variation for three shot samples, then "minor" covers pretty much all the variation shown among them. But if you think it is telling you something, then you are arguing that the variation I plotted is not within the limits of being "minor". Then all that remains is testing to discern if they are revealing something.