Pressure flats

[tomswede]

Some people do testing to find a pressure or velocity “flat” where the charge weight vs velocity curve flattens. This makes no sense to me, what physical process would cause this? More fuel more velocity until the very max. Anybody have any explanation?

 

[dcali]

There isn't one. It's a myth. Pressure increases with charge weight nearly linearly at the small increments we deal with. Technically, there is a curve to it, but not the "flat" that you hear people talking about. I've never seen it at least, and the theory doesn't show any reason for it to be there.

 

[glockaholic]

There isn't one. It's a myth. Pressure increases with charge weight nearly linearly at the small increments we deal with. Technically, there is a curve to it, but not the "flat" that you hear people talking about. I've never seen it at least, and the theory doesn't show any reason for it to be there.

I use to find the velocity flats when I had an older Chrono. I use to bet my bullets on them. They disappeared when I bought the LabRadar.

 

[Alex Wheeler]

They are there with the labradar. Why I have no idea. I look for them with hunting rifles. I run everything in ladder format, which makes those spots obvious.

 

[dkhunt14]

I have seen it also, most times not as obvious. What's nice is if those velocities print same point of impact at 1000 yards. Matt

 

[jimbires]

I see them too , using a labradar . that's a nice wide one Alex shows above , some are not that wide . I did a fair amount of playing with this flat spot this past summer . for a hunting rifle out to a couple hundred yards you can get on a load pretty quickly . here is a link to me working loads by watching the flat spot .

https://forum.nosler.com/viewtopic.php?f=4&t=35890

 

[Depart123]

If you shoot groups that have enough shots to be statistically significant and the powder graduations are sufficiently close together, you will see an obvious spot where SD's tighten up and velocities are also similar. As I understand it, it is because the powder/bullet combo has reached optimal pressure and powder burn efficiency.

Beyond this point, the total weight of the ejecta causes further pressure spiking and the pressure curve becomes erratic. None of this is set in stone, however, because when using a powder that isn't in the optimal burn rate to begin with weird stuff can happen.

But the whole idea that this can be done with a single round at each charge is, I believe, a myth. The variability of each individual round is greater than the average of the graduations themselves. if you find a flat spot this way, it is random chance.

 

[dcali]

Ok, I’ll walk back a bit. It seems like there could be some sort of coating or other subtle effect that would theoretically allow for a flat spot. I still think what people see is simple statistical variance. I’ve never seeen it when trying to fit a curve to multiple shots. There’s always more noise than signal- way too much to say that it’s not just random chance.

 

[Alex Wheeler]

If you shoot groups that have enough shots to be statistically significant and the powder graduations are sufficiently close together, you will see an obvious spot where SD's tighten up and velocities are also similar. As I understand it, it is because the powder/bullet combo has reached optimal pressure and powder burn efficiency.

Beyond this point, the total weight of the ejecta causes further pressure spiking and the pressure curve becomes erratic. None of this is set in stone, however, because when using a powder that isn't in the optimal burn rate to begin with weird stuff can happen.

But the whole idea that this can be done with a single round at each charge is, I believe, a myth. The variability of each individual round is greater than the average of the graduations themselves. if you find a flat spot this way, it is random chance.

You are making the assumption I base my comments from one test? I cant count how many ladders at 1k I have shot and what you learn shooting so many tests at distance is very valuable. I recommend it. We see it in the dasher, 100s of ladders. 3 shot per charge ladders in the heavy barrels. The more I do this stuff the more I see certain phenomenon I read on the internet can't or don't happen.

 

[dkhunt14]

If you shoot groups that have enough shots to be statistically significant and the powder graduations are sufficiently close together, you will see an obvious spot where SD's tighten up and velocities are also similar. As I understand it, it is because the powder/bullet combo has reached optimal pressure and powder burn efficiency.

Beyond this point, the total weight of the ejecta causes further pressure spiking and the pressure curve becomes erratic. None of this is set in stone, however, because when using a powder that isn't in the optimal burn rate to begin with weird stuff can happen.

But the whole idea that this can be done with a single round at each charge is, I believe, a myth. The variability of each individual round is greater than the average of the graduations themselves. if you find a flat spot this way, it is random chance.

If you look at Alex's test above, I agree that shot 2 is errant. But when You get to the middle at 85.7 to 86.6 it is pretty flat. That's almost a grain of powder and and four shots within 7 feet a second. Even with slight error if you loaded in the middle, I bet it would be a good load. When I do it I also put a target at 1000 and see where the bullets land. Most times the vertical is good there. Where they hit on the target also helps verify if the velocities are correct. If I had a doubt I would repeat it. I would then load three of each and shoot them at 1000 verifying tune. Matt

 

[retired]

( to try and keep up with tom ?)

So let me get this straight. You went outside and collected actual factual data? Why would anyone do that?

Tom

 

[Alex Wheeler]

If you look at Alex's test above, I agree that shot 2 is errant. But when You get to the middle at 85.7 to 86.6 it is pretty flat. That's almost a grain of powder and and four shots within 7 feet a second. Even with slight error if you loaded in the middle, I bet it would be a good load. When I do it I also put a target at 1000 and see where the bullets land. Most times the vertical is good there. Where they hit on the target also helps verify if the velocities are correct. If I had a doubt I would repeat it. I would then load three of each and shoot them at 1000 verifying tune. Matt

Matt, it is errant but its not. Let me explain. I have shot a lot of ladders, multiples in the same rifle and compared them with other ways of tuning in the same gun. Those spots that look errant are areas of high ES. If you were to load 10 of that charge you would see that high velocity again, but you would also see some 40 fps slower too. It just as well could have went slower than the previous shot.
FYI, when I do any test the powder charge is weighted to the kernel and the rest of the load is done like I was to reduce variables.

 

[dmoran]

There isn't one. It's a myth. Pressure increases with charge weight nearly linearly at the small increments we deal with. Technically, there is a curve to it, but not the "flat" that you hear people talking about. I've never seen it at least, and the theory doesn't show any reason for it to be there.

We all have our own inputs, experience, and opinions/theory, from which I have to say my input is in strong opposition of yours. My input is based on both chronograph and pressure trace resultants of the many charge increment ladder test that I have conducted over the last 13-years (switched to ladder testing in 2005).
In my experience, pressure/velocity does not linearly fallow charge increments. Most always see dominance in pressure/velocity curves over the extent of a couple grains of powder charges incrementally (with similarity of Alex's ladder tested velocity data he posted above).
Donovan

 

[dcali]

We all have our own inputs, experience, and opinions/theory, from which I have to say my input is in strong opposition of yours. My input is based on both chronograph and pressure trace resultants of the many charge increment ladder test that I have conducted over the last 13-years (switched to ladder testing in 2005).
In my experience, pressure/velocity does not linearly fallow charge increments. Most always see dominance in pressure/velocity curves over the extent of a couple grains of powder charges incrementally (with similarity of Alex's ladder tested velocity data he posted above).
Donovan

I shouldn’t have used the word myth. What I can say is that basic (possibly too basic) interior ballistics theory does not provide an explanation. And that my own experience has not lead me to anything I would call a flat spot, but rather a vaguely linear, noisy, continuous increase.

 

[retired]

burn rates for powder are very pressure dependent. hit a burn rate sweet spot and see little pressure difference with small charge changes. most effect/affect in the normal operating pressure . go low and get lost. go high and may see spikes vs bands.

I shouldn’t have used the word myth. What I can say is that basic (possibly too basic) interior ballistics theory does not provide an explanation. And that my own experience has not lead me to anything I would call a flat spot, but rather a vaguely linear, noisy, continuous increase.

 

[Mozella]

Here's a chart of some 6.5mm Creedmoor testing. The MV's at each charge weight are an average of five shots; i.e. this was a 50 round test.

The best groups were at a charge weigh of 42.1 and this promising charge weight was confirmed later on with additional testing and confirmed again at an F-Class match. You can see that there is no "flat", but there is a slight deviation from the linear trend line where the MV plot dips down showing a slightly less than expected velocity change corresponding to some sort of "sweet spot".

What does this velocity dip mean in the overall scheme of life? I'll leave it to the forum members to argue about it.

 

[Depart123]

You are making the assumption I base my comments from one test? I cant count how many ladders at 1k I have shot and what you learn shooting so many tests at distance is very valuable. I recommend it. We see it in the dasher, 100s of ladders. 3 shot per charge ladders in the heavy barrels. The more I do this stuff the more I see certain phenomenon I read on the internet can't or don't happen.

I wasn't referring to you at all sir, I'm sorry if I gave that impression. The single shot method has been discussed elsewhere on this forum, it is those discussions I was referencing. I should have been more clear.

 

[dcali]

Here's a chart of some 6.5mm Creedmoor testing. The MV's at each charge weight are an average of five shots; i.e. this was a 50 round test.

The best groups were at a charge weigh of 42.1 and this promising charge weight was confirmed later on with additional testing and confirmed again at an F-Class match. You can see that there is no "flat", but there is a slight deviation from the linear trend line where the MV plot dips down showing a slightly less than expected velocity change corresponding to some sort of "sweet spot".

What does this velocity dip mean in the overall scheme of life? I'll leave it to the forum members to argue about it.

View attachment 1031940

This is what I'm used to seeing - and if you plot all the shots, rather than just the averages, I'm going to guess that picking out that slight "dip" is nearly impossible.

 

[Alex Wheeler]

Here's a chart of some 6.5mm Creedmoor testing. The MV's at each charge weight are an average of five shots; i.e. this was a 50 round test.

The best groups were at a charge weigh of 42.1 and this promising charge weight was confirmed later on with additional testing and confirmed again at an F-Class match. You can see that there is no "flat", but there is a slight deviation from the linear trend line where the MV plot dips down showing a slightly less than expected velocity change corresponding to some sort of "sweet spot".

What does this velocity dip mean in the overall scheme of life? I'll leave it to the forum members to argue about it.

View attachment 1031940

Id be interested if you have done similar charts with Hogdon powders? RL powders do seem to be pretty linear in my experience. One reason I rarely use them in hunting rifles.

 

[Berger.Fan222]

From a statistical standpoint, one would need to shoot a number of shots for each step in charge weight. This allows computing both a mean velocity and an uncertainty in each charge weight. Then when one looks at a graph of the plotted velocity vs. charge weight, the issue of a possible "flat" does not depend on several points being below the best fit line, but rather whether the computed uncertainties (error bars) overlap with the line.

A more sophisticated (and rigorous) approach would be to fit the data (velocity vs. charge weight) to a third degree polynomial and determine if the uncertainty in the coefficient of the cubic term is significantly smaller than the cubic term itself. If it is, then the cubic term IS significantly different from zero, which would be solid evidence that an apparent flattening is not the result of a small sample size or other statistical anomaly.

I've done a lot of work over the years addressing the question of whether given measured data sets have a constant first derivative: is the data linear, or is there clear evidence for a non-linear trend? The purported "flattening" in a velocity vs. powder charge curve requires not only a non-linear trend, it requires the first derivative to first decrease and then increase again. Compelling evidence for this will usually require at least 20 data points.