Chronograph numbers

[scatterbrain]

RC 20, you assumed that I shoot 10 rounds in one quick motion, and you assumed that I take 10 shoots to hit a target.
If you would try a ten shoot group you will find a tight group some where near the center of your overall group, that is where your ammo hits, that is where your rifle is shooting. The group will look like your data, several close numbers, and both high and low outliers, but with 10 you will see the center of your group. The extremes won't point that out, and a three shoot group won't either.
By lowering my SD, and it took paying attention to everything assumed, my groups will run in the .25 MOA.

[RC20]

Ok, guilty.

Equally then my 9 shots work as well though for hunting I don't care about SD and ES of 60 gets me 1 inch or a tad lower.

I never shot any animal off a bench (well I did rest on the hood of my Pickup once)

[Jim Watson]

Equally the factory 270 that was 200 fps off and was way off on the target also told me something. It was not typical. It was wild. For typical I can throw it out.

You can throw it out if you are writing a gunzine article.
But if you come across an outlier like that in a match or when lined up on a trophy buck, it is all yours.

[hounddawg]

My first experiment with the Shotmarker is causing me to rethink the ES/SD thing. Two ten round groups is hardly proof, but my early evidence is indicating that shots fired at close to the same velocity are impacting at close to the same vertical POI at 300 yards, which rules out my barrel harmonics theory. Next week I want to run a more detailed test with 4 ten round groups and compare my optical chrono to the Shotmarker numbers. I found out today I can really secure the SM target to our range target stands really securely and am curious to see if the numbers correlate.

[RC20]

You can throw it out if you are writing a gunzine article.
But if you come across an outlier like that in a match or when lined up on a trophy buck, it is all yours.

The point was, factory or hand load, it was not representative (or I hope factory and no hand load I have done was that bad).

And testing is where you can throw it out (and I was testing), competition you make darned sure to double check your powder levels.

In this case (pun) I wanted to see what factory velocities were. 200 fps low does not tell me that. throwing it out does.

Data is not a blind thing. Gathering it serves a purpose. In this case it was not to do quality control check on factory but see what velocity they load to.

In my case I don't mess up in the above 2800 fps area in any of my guns. It was interesting to see that the factory 130 gr did indeed meet the velocity listed for those loads (3000 something). That is a cross check with my Chronograph as well.

[RC20]

I have some data on a good load and the results. As expected, the Chrono data backed up the results.

In this case at 105 yards, I put 10 rounds of 6.5 x 47 into a half inch. One raged hole (granted a larger one than the term usually means)

As it was all the same load and no adjusting on the COAL, I took data on all 10 rounds.

ES: 35 SD: 12 (Average Velocity was 2660.

As Hounddawg says, its the group that counts but if you want data that gives you a half and MOA, then that is what it looks like.

The numbers reflect good quality control, the testing found the spot it likes.

Now if the hole was 1 inch, then you know your quality is good, its just the velocity is not in the node the gun shoots well in.

I have crossed more than one load powder wise to match a velocity and it shot well plus or minus a bit at that velocity.

35 fps is pushing the limits as to what you can achieve (25 fps being implausible) and the SD at 1/3 the ES speaks for itself reflecting a good load.

I can't say enough about the 6.5 for that type of testing (with a very nice target trigger). It allows you to virtually eliminate recoil aspects and focus on the sight picture and the squeeze. The NF scope with the dot is an assist as you can hold and see a very small spot easily.

[Unclenick]

Meant to get back to this sooner.

The issue with ES vs SD is that SD varies less. If you shoot either nine or ten shot groups or strings, the average ES will be about three times the SD, but (within 95% confidence) 21 out of 22 groups or strings will have the ES vary about ±25%, while the SD will vary only about ±8%. So if you want a number that is representative of what to expect from future nine or ten-shot groups, you get better accuracy and consistency multiplying the SD by three than you do from using the ES number directly. Another way to look at it is, if your particular group happened to include an outlier, multiplying the SD by three lowers the effect that one outlier has on your result by a factor of the square root of nine or ten, whichever number of rounds you used.

Note that the factor of three only applies to nine or ten shot samples. For five-shot groups the multiplier is two-and-a-third. For thirty-shot groups, the multiplier is four. This is in the plot I posted earlier of group size or velocity extreme spread growth with the number of shots in a sample.

As far as the group center goes, it moves around from group-to-group too. The SD of that change in mean value is called the standard error and is equal to the SD divided by the square root of the sample size. In other words, for any given SD, the center moves around more for small groups with that SD than for large ones because that SD divided by the square root of the sample size is a smaller number for large samples.

[RC20]

And I thought my brain hurt before.

[ballardw]

Originally Posted by RC20

And I thought my brain hurt before.

Wait until you have explain to someone why a "bell curve" from a Cauchy distribution is not the same as "bell curve" from a Normal distribution and the general rules based on the Normal curve no longer apply.

[Bart B.]

After you've found an accurate load, will someone else shoot it to the same level with your rifle and ammo?

[Metal god]

And I thought my brain hurt before.

Oh no I got this !

[hounddawg]

The obsession with numbers gives me a chuckle. Last week at my range one of our regulars was "testing" ammo. Now keep in mind this guy is one of our top shooters and and is usually in the top three places in both mid and long range. As I was waiting on cold range and unpacking I asked him how it was going. He was all upset because something was just not right, looking at his target he was getting .5 to .75 MOA 10 round groups at 800 yards. I pointed out his groups were fine, he said yeah but my SD is way too high. I asked him if the next match was a chrono match to see whose ammo had the best numbers or would we be shooting at targets. He just gave me a puzzled look. For whatever it is worth I think he is still all worried about his chrono numbers

[Unclenick]

It applies to group size stats as well. Or should I chuckle because those are just numbers, too?

The underlying point is that if you are going to measure shooting results, you are going to get numbers. The question is, what do you want to do with them or what do you want them to tell you? That's what should drive your choices in stats.

For example, I've frequently used velocity SD as an indicator of ignition consistency. By itself, it may make little to no difference at short ranges or at a range where the trajectories cross, especially not off the bench. But in position shooting or in cold weather, that may change. So consistency seems desirable.

[hounddawg]

well Nick like I told "John" when they start having chrono matches I will start sweating the ES/SD numbers. Until then I just worry about the groups size numbers. If I could perform as consistently well in competition as "John" does I would not give a darn if my ES was 4 digits. For the record I don't sweat the shininess index numbers of my brass either.

[rclark]

if your particular group happened to include an outlier, multiplying the SD by three lowers the effect that one outlier has on your result by a factor of the square root of nine or ten, whichever number of rounds you used.

Makes sense. I got around that by putting my 15 shots in a spreadsheet and then 'eye ball' the data (look at min, max) ... If all are say within 10fps, but one is 20fps, I just delete it for the ES value that I'll commit to my load table.

[hounddawg]

By itself, it may make little to no difference at short ranges

have you considered barrel harmonics into your theories ? Here is a 1st load test of 68 gn Bergers. It is pretty obvious even at 100 yards that the velocity ES has little to do with the bullets point of impact. Look at 27.3 ad 27.8 both with low ES numbers and yet each had verticals of over .5 inches

[edit] might just be my public school education but I cannot find any correlation between ES/SD and group sizes or group verticals at all [/edit]

Which load would you choose to develop? 27.3, 25.8 or 28.0 ? I ended up going with 28.1 gns when the composite of 28.0 and 28.2 showed a POI of .402 height with a 95% CEP of .312 . That allows subtle changes in powder charge weight or temperature variations to have a minimal effect on the overall POI

[Hanshi]

Ten shots - some pundits claim 7 shots is statistically as valid - would be more than adequate, IMHO. However I've found 5 shots tell me all I really need to know. Five shot groups are cheaper than 7 or 10.

[hounddawg]

two is all you really need for elimination, three at the most. If the first two are beyond what group size (or ES if that is your thing) you are striving for then more shots will not bring them closer together

[RC20]

Wait until you have explain to someone why a "bell curve" from a Cauchy distribution is not the same as "bell curve" from a Normal distribution and the general rules based on the Normal curve no longer apply.

I am going to quit reading this stuff if you keep that up!

[RC20]

The obsession with numbers gives me a chuckle. Last week at my range one of our regulars was "testing" ammo. Now keep in mind this guy is one of our top shooters and and is usually in the top three places in both mid and long range. As I was waiting on cold range and unpacking I asked him how it was going. He was all upset because something was just not right, looking at his target he was getting .5 to .75 MOA 10 round groups at 800 yards. I pointed out his groups were fine, he said yeah but my SD is way too high. I asked him if the next match was a chrono match to see whose ammo had the best numbers or would we be shooting at targets. He just gave me a puzzled look. For whatever it is worth I think he is still all worried about his chrono numbers

Thank you, I needed the laugh.

On a practical note, I found a totally full 1 lb jug of Varget with a hand label with a ? on it. Ok, its the right size and color, but is it really Varget?

Hmm, what to do. I am far from desperate as I have at least 5lb of Varget but.....

Mixing with other is a bad idea until you know. Ponder.

Ok, I have been doing quite a bit of 33.6 gr (known Varget) with Lap 120-L bullets so I have a good FPS Data set.

Load up 5, see how they compare to the data set. Close and yes its Varget almost for sure, but also that sub batch test will tell me how close top the known Varget it performs at and does it need to be adjusted +/-?

Use it up and carry on.

[Unclenick]

Just to add to Houndawg's point about velocity at 100 yards, with his bullet going 3000 fps, a 100 fps velocity change amounts to about 0.17 moa of vertical change in POI from a perfectly rigid gun.

[hounddawg]

bullet going 3000 fps, a 100 fps velocity change amounts to about 0.17 moa of vertical change in POI from a perfectly rigid gun

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

[Unclenick]

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.

[hounddawg]

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

[Unclenick]

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.