Is FPS change always linear?
- Thread starter Leo_B
- Start date
Not really, the more the pressure goes up the more it will impact most powders characteristics
Jeff Porter
Gold $$ Contributor
My experience says that is true over a range of the charge weights but not from zero to max. You need a certain amount of powder to overcome inertia to get the bullet moving through the barrel and to the target. Once you have that, the linear section applies until you reach near max pressures and then things get spicy and jump up in a non linear way. Just my experiences, lot of variables with powder burn rates, case capacity, bore size and bullet weights, but in general , there is a linear section in there. Problem is assuming you know where it starts and where it ends in your system.
Pressure curve are different by powder. Generally, greater case fill leads to higher pressure which in turn generates greater velocity. Aside from case fill, the amount of resistance on bullet (“neck tension”, crimp, chamber clearance, bore, bullet geometry, etc.) will also impact pressure. As the bullet travels down length of barrel the incremental velocity gain changes. The other variable is your actual barrel length compared to barrel length used for determining test data.
Not linear.
Not linear.
I should have qualified, is it linear in any given recipe found in any reputable reloading manual for the one specific load given, I.E. in this case it is linear from 9.5gr/2100fps to 12.4gr/2600 fps. So I'm wondering, without ciphering every load/fps for every recipe is that a normal progression with all manual listed loads?
Are you asking:
1) if” you could identify/graph the velocity curve of a known load through a barrel then could you assume that this same incremental velocity curve can be used as a constant/yardstick that can then be applied to other loads to determine a velocity over different barrel length
> At any point along the length of the barrel the bullet is gaining velocity at different rates. The incremental gain in velocity is greater at the beginning and diminishes along length.
Or
2) are you trying to predict velocity based on different charge weight within the known load recipe?
> At different charge rates there is different case fill resulting in different pressure resulting in changes in velocity. It’s not linear, but you can get an approximation.
All things being equal, each powder has a specific pressure curve and burn rate. Pistol powders are fast and tend to capture majority of velocity in a short length of barrel. Rifle powders are slower burning and tend to continue to deliver meaningful incremental velocity gains over a longer barrel.
Hopefully this helped … maybe I’m just throwing horse shoes
1) if” you could identify/graph the velocity curve of a known load through a barrel then could you assume that this same incremental velocity curve can be used as a constant/yardstick that can then be applied to other loads to determine a velocity over different barrel length
> At any point along the length of the barrel the bullet is gaining velocity at different rates. The incremental gain in velocity is greater at the beginning and diminishes along length.
Or
2) are you trying to predict velocity based on different charge weight within the known load recipe?
> At different charge rates there is different case fill resulting in different pressure resulting in changes in velocity. It’s not linear, but you can get an approximation.
All things being equal, each powder has a specific pressure curve and burn rate. Pistol powders are fast and tend to capture majority of velocity in a short length of barrel. Rifle powders are slower burning and tend to continue to deliver meaningful incremental velocity gains over a longer barrel.
Hopefully this helped … maybe I’m just throwing horse shoes
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In the load I looked at there are 5 velocity steps in 100fps increments, 2100-2600. For each 100fps increment the powder charge is 0.6gr greater than the prior. So it is linear. But is it linear throughout? If I took the base 9.4 and added 1.5gr. is that going to be 2350fps? And is the 0.3gr = 50fps going to be linear through the 2100-2600 range?
And part 2, most of my manuals give starting and max but no incremental steps like this Hornady. So if manual X has a load of 9.4gr/2100fps and 12.4gr/2600fps can it be presumed each 0.6gr increase to the 9.4gr starting load adds 100fps and it is linear?
It isn't that big a deal as I'll likely always only be using the first couple of increments, but just something that piqued my curiosity when looking at the detailed chart and got me wondering.
And part 2, most of my manuals give starting and max but no incremental steps like this Hornady. So if manual X has a load of 9.4gr/2100fps and 12.4gr/2600fps can it be presumed each 0.6gr increase to the 9.4gr starting load adds 100fps and it is linear?
It isn't that big a deal as I'll likely always only be using the first couple of increments, but just something that piqued my curiosity when looking at the detailed chart and got me wondering.
Coyotefurharvester
Silver $$ Contributor
Yes you will see a lower velocity at a starting load, higher velocity at max suggested load. What happens in between is less predictable because of many factors, including the chronograph(its error range) used and number of velocity samples collected. When load data lists steps(incremental charge weights) what is left out is 1. Is the data actual firing data(or math generated) 2. Number of rds used and often what the test barrel used. Just way to many variables to use load data for predicting velocity, why you need a method (drop over distance )or equipment to measure.
Okay - I understand your question.
Right off the gate: each rifle is different. Assuming you used exactly same ammo as the test rifle, your velocities at the min./max. charge weights will most likely differ. This can be due to different barrel length, different twist rate, difference in bore and chamber, etc.
Let’s focus solely on the ammo, specifically varying the charge weight. As you increase the charge weight you increase case fill. Typically, as the case fill increases so does the pressure inside the case upon ignition resulting in increased velocity. However, as you approach 100% case fill the pressure does not increase linear, but for lack of a better term exponential. For this reason as you approach 100% case fill or even beyond (compressed loads) pressure begins spiking.
At lower case fill levels the incremental velocity gain from measured increase in powder charge are gradual. However, this curve becomes more of a hockey stick as you approach case fill. For this reason you will observe reloaders gradually working up their loads as it approaches max charges in 0.1gr - 0.2gr increments.
In the normal range of loads shown in manuals most if not all powders will appear to be linear. In actuality this isn't the case but the deviation isn't usually detectable. At wider ranges where case fill and lower pressures are involved then things do deviate from linear.
One thing that can also deviate is if a powder designed for low pressure loads like pistol or shotgun powders are mistakenly loaded in a rifle cartridge then all bets are probably off since the deterrents used are not used with rifle pressures in mind.
One thing that can also deviate is if a powder designed for low pressure loads like pistol or shotgun powders are mistakenly loaded in a rifle cartridge then all bets are probably off since the deterrents used are not used with rifle pressures in mind.
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A chronograph is the only way to quantify results. If you don’t have one to shoot with whether yours or someone else’s you’ll never know what’s going to happen and you’ll never be able to figure out certain trends or anomalies that are going to happen.
dellet
Gold $$ Contributor
You can’t even count on published predictions to be correct unless you consider +/- 50 fps as precise.
What is pretty common is the way load data is done. For the most part there is a maximum load and then a starting load that is roughly 10% less powder. Sometimes that will brake down linear, sometimes not. Stages or steps are often calculated more or less at 5% of previous charge.
If you’re interested you might look at Hodgdon rifle loads for something like a 308 Win. that will have full power, reduced recoil and subsonic loads. If your real lucky, you might find loads in all there categories using the same powder, but I doubt it. Possibly full power and reduced recoil. See if the numbers pan out.
If you work with a powder and bullet combination enough you will see your own pattern develop. In the Hornet and something like 1680, it might just end up .1 grain is 10 fps. But before you can see that, you need a load that consistently has a single digit ES.
Things like this are a good way to chase your tail and burn out a barrel finding a definitive answer. But it can be fun.
What is pretty common is the way load data is done. For the most part there is a maximum load and then a starting load that is roughly 10% less powder. Sometimes that will brake down linear, sometimes not. Stages or steps are often calculated more or less at 5% of previous charge.
If you’re interested you might look at Hodgdon rifle loads for something like a 308 Win. that will have full power, reduced recoil and subsonic loads. If your real lucky, you might find loads in all there categories using the same powder, but I doubt it. Possibly full power and reduced recoil. See if the numbers pan out.
If you work with a powder and bullet combination enough you will see your own pattern develop. In the Hornet and something like 1680, it might just end up .1 grain is 10 fps. But before you can see that, you need a load that consistently has a single digit ES.
Things like this are a good way to chase your tail and burn out a barrel finding a definitive answer. But it can be fun.
As stated above, the short answer is, yes, they can be considered linear over that short range of loads. They really aren't but the difference is so small you won't see it. Your extreme spread will likely be more than that difference.
Can you imply that your load will match that velocity. No. One time I was 'lucky' enough to have the same model rifle that Sierra used to compile their load data for a cartridge. I even bought the same brand brass, primers, and bullets. Loaded them to the OAL listed in the data. My velocities were >50fps different than the manual. One load was more than 100fps off.
The manuals are just a starting point. There is a reason why it is suggested you start low and work up a load, watching for signs of excess pressure.
Can you imply that your load will match that velocity. No. One time I was 'lucky' enough to have the same model rifle that Sierra used to compile their load data for a cartridge. I even bought the same brand brass, primers, and bullets. Loaded them to the OAL listed in the data. My velocities were >50fps different than the manual. One load was more than 100fps off.
The manuals are just a starting point. There is a reason why it is suggested you start low and work up a load, watching for signs of excess pressure.
ELR LVR
Silver $$ Contributor
No, not linear
More exponential the more your vessel is filled
it may not show much for even a grain more powder, until a certain point, then shoot up dramatically
----
OR even go down once the case is filled, I have seen that as well
Manual A gives entries for starting load 9.4gr 2100fps. Then increments and shows you 2200, 2300, 2400, 2500 and finishes with max load of 12.4gr 2600fps. Every increase of 100fps is 0.6gr higher than the last.
Manuals B, C, D and E give two entries, starting load 9.4gr 2100fps and max load 12.4gr 2600fps.
What I am trying and failing to ask is when looking at loads that aren't in manual A and I only have min/max load data, and I start with 9.4gr and work my way up to 10.9gr, the midway point of 9.4/12.4 can I presume that like the load with details in manual A this load is also going to be midway in fps from 2100 to 2600? And another load that's 2000 to 2400fps min/max, if I load exactly the middle amount of powder can I presume it is 2200fps, the midpoint of the given speed? I know, get a chronograph, but until then for any random load in any manual that only gives min/max if you use exactly the middle powder charge do you presume it is exactly the middle speed of what they give? It doesn't really matter what it is, it's a safe load, worked up to from the minimum load and way below the max load, but just curious if that is a semi-reliable speed estimate to use until a chronograph is ok with mr. grumpy checkbook.
Manuals B, C, D and E give two entries, starting load 9.4gr 2100fps and max load 12.4gr 2600fps.
What I am trying and failing to ask is when looking at loads that aren't in manual A and I only have min/max load data, and I start with 9.4gr and work my way up to 10.9gr, the midway point of 9.4/12.4 can I presume that like the load with details in manual A this load is also going to be midway in fps from 2100 to 2600? And another load that's 2000 to 2400fps min/max, if I load exactly the middle amount of powder can I presume it is 2200fps, the midpoint of the given speed? I know, get a chronograph, but until then for any random load in any manual that only gives min/max if you use exactly the middle powder charge do you presume it is exactly the middle speed of what they give? It doesn't really matter what it is, it's a safe load, worked up to from the minimum load and way below the max load, but just curious if that is a semi-reliable speed estimate to use until a chronograph is ok with mr. grumpy checkbook.
I've been wondering this myself for a few weeks.
I'm beginning to think that just perhaps it is:
From about 30% to say 95% of max recommended loads. But that's with mid burning powders such as varget.
Very fast or slow powders I think not.
Plan to test the idea with GRT soon. Linux refuses to run it so need to borrow a windows PC to do it.
I'm beginning to think that just perhaps it is:
From about 30% to say 95% of max recommended loads. But that's with mid burning powders such as varget.
Very fast or slow powders I think not.
Plan to test the idea with GRT soon. Linux refuses to run it so need to borrow a windows PC to do it.
Mostly.
Mostly.
My testing shows a strong linear relationship. For example, the chart below, with each test shot being .4gr higher than the last. This is single-shot data and would be smoother with multiple shots averaged at a given charge weight. I've done multiple tests like this with rifles, different powders, always linear. Don't know about handguns.
Having said that, I'm told that the curve will flatten near the top of the permissible charge weight. I don't push things high enough to see that.
dellet
Gold $$ Contributor
Sadly charts are open to interpretation. The only part of that graph that’s linear. Is the top five shots. The rest shows why load density is important.
It would be easy to draw lines showing 1.2-1.6 grains difference resulted in 50 FPS change in velocity in a much more stair step pattern. If there is an accuracy node at any of those flat spots, it’s a nice place to be.
From 2550 to 2650 fps. It took 8 different charge weights 12.5 fps. While 2700-2800 fps 4 charge weights 25 fps.
There is a lot of information in that chart and it all points to the only correct answer to the original is “it depends”.
Or at least Thats my interpretation of the data.
Edit to add
Those top five shots will be what is presented in any load data that is published.
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