On one hand, it just begs the question why that spike up in velocity in the middle of the curve then the roller coaster down?
This load is N565 which is a very chunky extruded stick powder---I call the N 560 through N570 series of VV powders the chunky monkeys. They are very hard to measure accurately without weighing every charge, even then one can be off a little.
These curves were derived by averaging 4 shot groups. The paticular ES on these 4 shots was 26 with the high being 2950 and the low being 2924. The Standard deviation was 13.05. So it was not a stellar 4 shot group in terms of the quality of the ammo itself.
The ES on most other data points was in a range from 8 -14 with SD's ranging from 4-13. So it as a group was an outlier, indicating to me I could have gotten some bad charge weights in those 4 pieces of ammo. It also could reflect a very hot chamber that I did not allow to cool enough between shot strings thus affecting burn rate and velocities. Could be both.
Now, at a more fundamental level, you are really asking, why does a velocity curve flatten or
even go DOWN with increasing charge weight? That seems to violate our simplistic view of the laws of physics doesn't it?
But, as it turns out, the scientific and physics reason why we see these flat spots in velocity and even a downward move in velocity
as we shoot higher charge weights is no mirage, and not solely related to as some will claim insufficient data to be statistically significant. I believe it is real. I have attached a chart I randomly pulled off the internet of velocity vs. charge weight. The flat spots and even some decline with increase in charge weight is shown. You can find thousands of samples like this from shooters right here on LRH or on the internet. It is real. It happens. Here is why.
Here is an excerpt from Chris Long's White Paper on Optimal Barrel Timing Theory which is very much based on barrel harmonics.
Chris is an electronics engineer who is very versed in physics and sin waves common to electronics engineering.
http://www.the-long-family.com/OBT_paper.htm
(This is not a secure sight but it works fine for me.)
The pressure pulse from the gasses in the chamber cause a traveling wave of stress that bounces back and forth along the barrel between receiver and muzzle, slightly changing the bore diameter in the process. Minimum dispersion of the shots will result when the rate of change of the bore diameter is at a minimum, and this dispersion will present the least sensitivity to load variations (charge, seating depth).
So, you have charge weights which fill a certain volume in the case, and are packed in a certain density in that case, and the powder you are using has a certain burn rate character which is also a function of volume, pressure, and density of packing, etc.
We cannot monitor chamber and barrel pressures along the length of the barrel dependent on how much of the powder is changed to gaseous state before exiting the barrel. That whole function depends on your bullet weight, your bullet seating depth, and the length of your particular barrel. But what we can monitor as a
proxy for pressure is muzzle velocity. So, this is all related to barrel harmonics and optimum barrel time for your bullet and charge weight. In compressed charges, some powder may actually exit the barrel unburned, especially in a short barrel.
The barrel harmonics from differing charge weights and bullet seating depths result in the sine wave variations up and down the barrel which moves the point of impact of the bullet as the barrel flexes and rotates in different positions at barrel exit of the bullet.
As Long mentions, the barrel undergoes expansion and contraction in frequency cycles as the bullet moves down the barrel. The sonic and pressure waves move back and forth in the steel with pressure in the chamber exceeding 60,000 psig. Because the speed of sound in stainless steel is about 5000 m/s, these waves move back and forth through a well bedded barrel multiple times before the bullet exits.
This affects pressure all along the barrel, waves coming and waves going, and therefore it affects velocity based on whether the bullet encounters pressure waves coming back or leaving the barrel and so the timing and velocity of the bullet leaving the barrel is affected in counter intuitive ways.
Therefore a small change in a charge could produce a counter intuitive failure to increase or decrease the velocity if the difference in the charge caused the bullet move through regions or zones of conflicting pressure interference in the barrel.
Here is another white paper which may prove of interest on this subject.
https://www.frfrogspad.com/loaddev.htm
There are some outstanding references to barrel harmonics work which I have noted in past posts here on LRH which you can search for. The whole subject was barrel tuners and why they work. Videos and white papers.
One of the best is Varmintal.com a Lawrence Livermore retired physicist who has performed finite element modeling analysis on barrels in models. Very good pictures of barrel harmonics in the process. Amazing stuff. Wet noodles?
Varmint Al's Eclectic Web Pages
Now, when you see a dramatic spike like the one you noted in my curve, its probably not all due to barrel harmonics and
may exaggerate the flattening and drop in velocity you will see by increasing charge weights in order to find the
"velocity nodes" where velocity flattens or even goes down some as charge weight goes up. It was more likely due to
bad charge measurement and or a hot chamber or both and some of the physics we discussed above.
One last thought. When you shoot a ladder test, you are looking for the spot where you get the least vertical dispersion in your shots on the target even though the charge weight goes up? Isn't that also counter intuitive in physics? I believe its the same reason you see flat spots or even decreases on the velocity graphs. Same phenomenon. Just different method to find the sweet spot where your loads will be most forgiving over a range of powder charge weights in your specific rifle, with your specific bullet,
for that specific powder. If you graph velocities from your ladder tests, I think you will see the same flat spots, not just smaller vertical strings on the targets themselves.
Now, aren't you glad you opened that can of worms?????
That's the answer to why the ladder test and the Satterlee tests work, and it explains as best I can why.
Now here's another LRH thread where this was similarly discussed and you will see some dissenting posters who don't believe this.
They say its not statistically valid. But you can make statistics say anything you want to. Statistically speaking on average every person in the world has one testicle and one breast, right??
No one said it burns inconsistent and then magically is consistent. But the same thing happens every time you run this test with any combo. You find spots where the velocity doesn't change even though the powder is increased. Sometimes for up to 1 grain of charge increase there is no velocity
I started this link on page 7 top......but do read the whole thread. Its very valuable discussion on the whole.
