A chart like that looks a lot different if you plot five shots at charge. Plot each shot not an average. I don't care what charts look like. As long as the groups are round and small I am happy.
Is FPS change always linear?
- Thread starter Leo_B
- Start date
Yep, close enough to linear. More data would provide a better graph. Those bumps could simply the result of variations such as case capacity or powder charge discrepancies.
The data points are visibly close to being linear. Formal analysis shows us that the coefficient of determination is .97, which means that the data is a very good fit to the line.
As mentioned, this is single shots, so we can't expect more-perfect linearity.
And yes, it does seem to settle out in the top five or so shots. The range goes from 80% fill to 97% fill.
Yes, which is why I mentioned it's single-shot. Having said, it's just the chart that came to hand. My data with multiple shots for each charge weight show similar linearity.
Agreed.
Just ES variance at low charge densities - or plain luck. But as mentioned charge weights go from 80 % to 97% fill, and one would expect greater variation at very low fill ratios.
I think for my purposes, a WAG of velocity until Mr. Grumpy Checkbook approves a chronograph, is using a linear progression from min to max load data in my manuals that only give min/max and not intermediate values. It doesn't really matter anyway, only that my gun likes shooting a given load and more importantly that I like shooting a given load. Since all loads will be in the bottom half of manual data listings I'm sure they'll be fine for fps and pressure.
What are you shooting that takes 10 gr charge? You don't need an exact numbers. Zero the rifle at 100 yards, then shoot a group at 200 & 300 yards. Make a drop chart. That should cover most hunting situations.
.22 Hornet
I also ran a load through GRT. A 223 Remington load using a 69 gn bullet and BL-C2 powder. Did a little different analysis than the analysis above. I looked at the differences, in PSI and FPS, between 0.2 gn increases in powder. All other parameters remained constant.
Results
1) There is a constant percentage increase in PSI of 2.8% between each 0.2 gn powder increase.
2) There is a constant percentage increase in FPS of 0.8% between each 0.2 gn powder increase.
3) While there is constant percentage increase in PSI, there is an increase in the absolute pressure difference between 0.2 powder increases as the total powder usage increases. Example: difference between 25.9 gn charge and 26.1 gn charge is 1219 fps; difference between 27.9 gn charge and 28.1 gn charge is 1601fps. This is something to keep in mind if modifying loads based on a linear assumption and increasing the total powder usage in a cartridge.
4) The muzzle velocity exhibits the same characteristics as the chamber pressure, but the changes are so small that they appear to be of a linear nature.
Edited to add PSI/Gn powder and FPS/Gn powder data.
Results
1) There is a constant percentage increase in PSI of 2.8% between each 0.2 gn powder increase.
2) There is a constant percentage increase in FPS of 0.8% between each 0.2 gn powder increase.
3) While there is constant percentage increase in PSI, there is an increase in the absolute pressure difference between 0.2 powder increases as the total powder usage increases. Example: difference between 25.9 gn charge and 26.1 gn charge is 1219 fps; difference between 27.9 gn charge and 28.1 gn charge is 1601fps. This is something to keep in mind if modifying loads based on a linear assumption and increasing the total powder usage in a cartridge.
4) The muzzle velocity exhibits the same characteristics as the chamber pressure, but the changes are so small that they appear to be of a linear nature.
Edited to add PSI/Gn powder and FPS/Gn powder data.
| Powder Charge | PSI | Delta | PSI % Diff | FPS | Delta | FPS % Diff |
| 28.1 | 59116 | 1601 | 2.8% | 3120 | 24.0 | 0.8% |
| 27.9 | 57515 | 1558 | 2.8% | 3096 | 24.4 | 0.8% |
| 27.7 | 55957 | 1511 | 2.8% | 3072 | 23.9 | 0.8% |
| 27.5 | 54446 | 1482 | 2.8% | 3048 | 23.4 | 0.8% |
| 27.3 | 52964 | 1438 | 2.8% | 3025 | 23.9 | 0.8% |
| 27.1 | 51526 | 1400 | 2.8% | 3001 | 23.1 | 0.8% |
| 26.9 | 50126 | 1365 | 2.8% | 2978 | 23.2 | 0.8% |
| 26.7 | 48761 | 1329 | 2.8% | 2954 | 23.1 | 0.8% |
| 26.5 | 47432 | 1294 | 2.8% | 2931 | 22.9 | 0.8% |
| 26.3 | 46138 | 1256 | 2.8% | 2908 | 22.7 | 0.8% |
| 26.1 | 44882 | 1219 | 2.8% | 2885.6 | 22.6 | 0.8% |
| 25.9 | 43663 | 2863 |
| Powder Charge | PSI/ Gn | FPS/Gn |
| 28.1 | 2104 | 111 |
| 27.9 | 2061 | 111 |
| 27.7 | 2020 | 111 |
| 27.5 | 1980 | 111 |
| 27.3 | 1940 | 111 |
| 27.1 | 1901 | 111 |
| 26.9 | 1863 | 111 |
| 26.7 | 1826 | 111 |
| 26.5 | 1790 | 111 |
| 26.3 | 1754 | 111 |
| 26.1 | 1720 | 111 |
| 25.9 | 1686 | 111 |
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Of course. Labradar LX.
Reading back through this thread I think you are trying to do something that can be a problem. I'm not sure what you are loading for but trying to compare data from one manual to another can be a problem. One of the first things to keep in mind is that each manual list it's maximum safe load. This may or may not be a load at SAAMI maximum average pressure. It depends on how the data is published. Also even with same spec components and powder they are different lots so lots of variables change. Some manuals give velocity from the SAAMI test barrels and others (Hornady) give velocities in an actual gun for the SAAMI test pressure loads so their velocities usually are slower. than others, and possibly more realistic.
Sometimes it flattens and sometimes it spikes. To this point, two things might be happening near maximum loads. The first is that as the case fill and the powder begins to compress the ignition/combusion rate of the powder may change. Also depending on the chamber pressure and powder combination the deterrent effects may also change at pressures higher than the powders were expected to operate.
@Pirate69
Great post. Internal balistics is an interesting subject.
"4) The muzzle velocity exhibits the same characteristics as the chamber pressure, but the changes are so small that they appear to be of a linear nature."
Yes, I noticed that too. It's a small gradual change.
Unfortunately only had a small amount of time to access the pc. Other wise i would have look into it further.
Still, if you want to guesstimate fps you can.
As im sure most will understand, it's just a program. There are other variables such as neck tension or cartridge capacity or neck tension that can create changes to psi or fps also.
Great post. Internal balistics is an interesting subject.
"4) The muzzle velocity exhibits the same characteristics as the chamber pressure, but the changes are so small that they appear to be of a linear nature."
Yes, I noticed that too. It's a small gradual change.
Unfortunately only had a small amount of time to access the pc. Other wise i would have look into it further.
Still, if you want to guesstimate fps you can.
As im sure most will understand, it's just a program. There are other variables such as neck tension or cartridge capacity or neck tension that can create changes to psi or fps also.
Nope. Not trying to compare manual 1 to manual 2. I'm asking if you are given min/max loads in a specific manual but nothing in between is the change in fps with interim loads linear. I clearly explained it in one of my posts, I thought, if min/max is 2200/2600fps with 9.0/10.0gr is 9.5gr 2400fps, the linear amount up the range. Can it be estimated as approximately that. Manual #5 gives every intermediate load recipe/data, the rest do not. Manual #5 tells you exact numbers at every level. The rest you have to make a WAG. So is using a linear progression reasonable for the WAG. I don't know how to ask it any more clearly.
You can assume the relationship is linear. I don't know if every manual with the data shows it to be linear but I know that several do. That is true for rifle. I suspect that some pistol cartridges may deviate more from linear because of the variability of case fill and pressures. I don't have enough experience to say for sure. However from a practical standpoint I would assume it to be linear.
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Most of the manuals, of my modest collection, don't show anything except minimum and maximum load and data. One of mine shows every 100fps interval, and a few with 50fps interval, between min/max. So it is crystal if the midpoint charge weight is also midpoint fps (linear) or not. Thus my question if working with a load from another manual is linear estimation reasonable. I'm going with it is close enough for government work and being between the specific manual's min/max loads it is safe to load.
I agree with your comment. I use the programs to get an idea of what charges I need to get to a desired velocity range (where a node should be) and screen for potential pressure issues. The chronograph tells the final tale.
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