Velocity and pressure

jonnefudge

New member
Is the velocity of a certain load linear to the pressure. For example: quickload shows 2800fps at 60.000psi for a certain load at 45 grains with a 26” barrel. But particular rifle (also 26”) achieves that 2800fps at 49grains. Does this mean that you will still have 60000psi even though you load more powder?
 
Absolutely not! Every rifle is different, there are slow barrels and fast barrels. The load data that quick load listed is a computer generated rendition that will show you where you should start if you put in the correct criteria to start with.

It shows a lot more than that but I'm trying to keep this short.


I watch for pressure signs when I start to work up a load.
I have one rifle in 30-06 that is tight and short chambered. It maxed out showing bad pressure signs at 2 gr over minimum. I never loaded it over that.

I have a 8mm Mauser that I can go over max with that doesn't show any pressure signs at all.

I don't pay attention to the fps I see as much as I do the pressure signs I see in the brass. My rifles and I stay healthier that way.

If your are not seeing any pressure signs in your brass at all and want to push the load a little, that's your choice.
The pressure your rifle will have depends on how tight the chamber and the barrel is.


There is no such thing as a pressure and velocity being linear, period, when it come to firearms. That's why it's called a pressure curve.
 
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Ok but it is hard for me to understand the physics in this equation. The pressure should have a very large part in the generated velocity. The temperature has a large impact on velocity and also on pressure. And if you are using the same everything except powder charge then the pressure curve should stay the same and therefore making the velocity a function of pressure??
 
Tis a complicated question. But I'm going to disagree with LE-28. If you're shooting the same bullet to the same speed with the same length barrel then you're getting pretty close to the same pressure even though the powder charge is different.

There are slow and fast barrels. I've seen as much as 130 fps difference between 2 barrels of the same length using ammo from the same box of handloads. The slower barrel, due to differences in dimensions and tolerances was operating at lower pressure, and slower speeds.

But if you have reached the max load listed in the manual, and are still well below the max speed listed it is NOT a good idea to increase the powder charge in order to match the expected speed. Technically, it should be safe in that rifle, but if those loads got into another rifle would be an overload.

BUT...

It is not unusual for a manual to show 47 gr of powder as a max charge @ 2800 fps. If YOUR rifle reaches 2800 fps at 45.5 gr, then your rifle is at the max pressure at only 45.5 gr of powder, not 47 gr. Another rifle might need 47 gr to reach the max pressure and 2800 fps. I've had this happen before too and is why I like to use a chronograph to check hand loads.

I know there are guys who push the envelope and load above listed max loads. It is not something I will do. I'll load right up to the max listed load if I'm not exceeding the expected speeds. But even if I'm much slower than expected I won't load any more powder.

I might try a different powder if I'm not getting the speeds and accuracy I want. But I don't load above listed loads. Looking for pressure signs is not a good method. None of the traditional pressure signs even show up until you exceed 70,000 psi. By the time you see them you're WAAY over.
 
I doubt that for a given bullet/cartridge/barrel length, the velocity is a pretty close indicator of pressure. My doubt is based on the fact that the peak pressure is reached before the bullet has traveled more than a few inches in the barrel; and even at the point where peak pressure is reached, the velocity is only half or less of the muzzle velocity (in the case of a rifle).

If velocity is a good indicator of pressure in those instances, it seems the characteristics and dimensions of the rifled part of the barrel (other than length) would have little affect on velocity. Just seems doubtful to me, but I don't really know.
 
I go with starting low and working up. If I encounter pressure signs before a max listed load, I stop, pull back.

I really don't care about velocity, though purported to be an indicator that your load is now gone sour (no more speed but sticky bolt? )

The indicators I look for are primers (not definitive but worth a look) marks on the back of the casing (wipe) - primers are an alert but not a given, sticky bolt is solid you are past things a bit, marks on the backs of the cases the same.

I have looked at my manuals and have seen several grains of powder difference between my two prime (Sierra and Horandy). I got with the lowest max and if I am exploring a higher accuracy node, proceed with caution above that.

My sense of Quickload is its a good guide but not something you would go to max on. You could have a hotter batch of the listed powder.

It also depends on accurate data input. A mistake can be misleading.

I would consider it another source along cross ref with reloading manuals.

Experienced people can extract loads out of it I believe that are not normal, but I am not one of them.
 
Ok but it is hard for me to understand the physics in this equation. The pressure should have a very large part in the generated velocity. The temperature has a large impact on velocity and also on pressure. And if you are using the same everything except powder charge then the pressure curve should stay the same and therefore making the velocity a function of pressure??

Your logic is sound, but only to a point.

Say you had a good load that was 3000 fps using a brand of brass with 60g of water capacity and then you changed to a different brand that has 65g of water capacity.

By your logic a small increase in powder will be required to get back to 3000 fps due to the pressure lost by going to a larger capacity case. This is good thinking.

But it won't be linear. It most definitely WILL NOT be 5 grains, (the change in case volume) adding 5 grains would likely put you in proof territory or beyond.

This is one of many reasons we start low and work up to faster loads, to account for the variables like case capacity, primer energies, etc.
 
Ok a important part of the question is that everything is the same in the load and barrel length expect powder charge. We also know that certain barrels is faster which makes the question relevant. If I get 2800fps a cold day and 2870 warm day on a specific load then the pressure should also be higher. And quickload tells me that you get 2800fps with a certain powder at 58000 psi and 2870 at 60000 psi. Shouldn’t this mean that you can calculate pressure through velocity? And shouldn’t this also men that the velocity itself tells me when I’m at 60000psi? Is this also meaning that the velocity for load development for another rifle tells me when I’ve reached 60000psi even though it requires more or less powder?

I guess it boils down to if the pressure curve changes between different rifles of the same model with the same bullet jump to the lands?

I’m from Sweden so English is not my native language so sorry if it is hard to understand my question/thesis.
 
Without getting too techy...

The final velocity of the bullet is the sum total of the energy gained by it from the burning of the powder.

Mathematically, that energy equals the area under the pressure curve. It is not a function of peak pressure but average pressure.

In no way does the final energy predict the peak of that curve. Just like you can have a 100sq-ft room that is 10x10 or 20x5 or 1x100... you can have a pressure curve with any given area that has an infinite possibility of shapes.
 
It will be closer to 2.5 grains to make the pressure match, and probably just closer to 2.25 grains to get the velocity to match.

You do need to understand a little basic physics here. On the plus side, if you don't like physics, this is pretty easy when you are just trying to get a velocity match from two guns with the same barrel length using the same bullet and powder and case and primer in the same chambering.

Note that most SAAMI pressure barrels are 24", so if you compare a 26" barrel's velocity to published pressure and velocity data, you have to allow for that difference.

At any given instant, the force pushing the base of the bullet in a gun barrel is the value of the pressure at its base divided by its cross-sectional area. When the force is high, acceleration is high and vice versa, but final velocity is the net result of the acceleration that occurs all the way down the barrel, and not just at the peak, which is why velocity does not give you a peak pressure number.

In physics, the concept of work is just force times distance. In English units, it is the distance in feet times the pounds of force that applied to move an object, so the unit of work is foot-pounds. In SI metric units it is newtons of force times the meters of distance over which that force is applied, so the unit of work is the newton-meter.

You may have noticed the English unit of work is the same as the English unit of energy: the foot-lb. Similarly, in the SI units, the newton-meter is a measure of energy equal to one joule. Indeed, when you look at projectiles, the kinetic energy they carry at the muzzle is exactly equal to the work put into moving them down the length of the barrel. For that reason, some like to think of kinetic energy as stored work or potential to do work. It is always equal to the amount of work the bullet can do parting the air and acting on its target.

When you calculate the kinetic energy of a bullet, it is the square of its velocity times half its mass. It turns out the velocity in that equation is the exact velocity the work equal to that kinetic energy will bring the bullet to when it reaches the muzzle.

So, if you have two bullets the same weight, and they reach the same velocity in the same length of barrel, the average force and therefore the average pressure behind them in these barrels must be the same, discounting any difference in friction. However, peak pressure is not average pressure. The peak pressure need not match in that situation, as explained next.

Below is a plot of two different powders producing the same velocity in the same barrel length. One is a fast powder; one is a slow powder. The slow powder has a heavier charge weight that produces greater total gas volume. That is why it keeps the pressure in the barrel up higher later in the bullet's travel. Because that higher late pressure provides more late barrel acceleration than the faster burning powder does, it makes up for producing lower acceleration at the pressure peak.

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So, velocity and barrel length tell let you deduce average pressure in the barrel but not a peak pressure value. Peak pressure is the one that is critical for safety. But what about the situation where you have the same chambering, the same case, primer, and powder and barrel length, but it takes more powder in one gun to reach the same velocity the other gun does with less powder? Well, like the slow powder in the example, above, the larger charge will produce higher late barrel travel and muzzle pressure that make up for acceleration not present at the pressure peak. So, the peak pressure in the gun requiring the heavier charge will be lower.

Any time you need more of the same powder to reach the same velocity from the same barrel length in a second gun with the same barrel length, the peak pressure in that second gun will be the lower of the two at that velocity. That is so, regardless of whether the extra powder was needed because your case capacity is greater or because your primer is weaker or because your gun's dimensions are just loose. But, conversely, if you need less powder to reach the same velocity with the same bullet in the same barrel length (say, your case had much less capacity than the original test case did), then you have higher peak pressure than was present for the databook author's load measuring.

One may be tempted to try to translate this to other chamberings. Note that other chamberings typically have different case lengths, and since barrels are measured from the breech face, bullets from, say, a 30-06, have a little less bullet travel in a 24" barrel than a 308 Winchester does. So keep the different barrel lengths in mind. Be sure you know how to extrapolate velocity difference from a different barrel length. QuickLOAD does this best, but you do have to tweak the arguments to get its output to match measured pressure and velocity from the factory data first. Absent measured load data to compare it to, you must, for safety reasons, assume it has an error margin and treat it only as an estimate.
 

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jonnefudge asked:
Is the velocity of a certain load linear to the pressure.

No! Never!

Absolutely Not!

At certain points in the development of a load from Starting Load to Maximum Load, the velocity curve may be nearly linear, but it is a mistake (sometimes a FATAL mistake) to assume the pressure vs. velocity relationship is constant throughout the published load data.

STOP - how can I make this clear to you? - STOP assuming Quickload is the be-all and end-all of reloading. At it's best, it is a GUESS.

Quickload MAY (but not always; it certainly did NOT in my circumstance) provide a Starting load, but recognize it is computational - not real world - data and so may not be safe.
 
Ok thanks for all the input. Just to be clear I am not some crazy reloader who tries to push dangerous limits. Anyone who reloads should always stay within safety margins. However, I think that in this case understanding the relation between pressure and velocity when EVERYTHING else stays the same actually makes hotter loads more safe as you can reliably measure velocity changes from powder temperature. Pressure signs on the other hand can be deceiving from what I understand.
 
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They can be deceiving, but it's not an either/or situation. Keep watching for pressure signs as well as velocity change checking. They may not indicate pressure accurately, but they do indicate that the brass or primer showing the sign is coming under stress and may be nearing its limit. It's not unusual to hear of someone getting pressure signs with one brand of brass, like Hornady or Federal, but it goes away when they switch to something harder, like Lake City.

Also, with loads that are not compressed, you will see pressure and velocity variation depending on whether the powder was over the flash hole or not. The SAAMI test method used with commercial ammunition includes manipulating the cartridge so powder falls over the flash hole and is carefully loaded so as not to disturb that before firing. If you have a range that allows a loaded gun to be aimed skyward, the simplest thing is to point the muzzle up and hit the butt with the palm of your hand and then level the gun slowly before firing. If you get in the habit of doing this, you are comparaing apples-to-apples every time.
 
Just to be clear I am not some crazy reloader who tries to push dangerous limits.


I don't think any of us here are implying that. The problem is when the data as HDWhit stated seems linear, it leads us to believe that if we add another half grain of powder that the velocity will continue to increase the same amount you have been seeing.

When the velocity curve starts to go horizontal you could still be adding a half grain of powder at a time and all of a sudden the velocity is showing only half the gain it was when it looked linear. Then the next half grain you are only see a fourth of the velocity gain you were seeing.

The pressure is still going up and up with diminishing returns on the velocity curve.

You apparently have a chronagraph so when you do the math on what looks like a linear curve and see you start loosing ground on velocity, according to your math, that's when the velocity curve is flattening out and you can be assured the pressure is going nowhere but up.

That is where you need to stop your ladder and consider that powder charge is as high as you dare go. It doesn't matter where your velocity lands according to what a loading manual says, if your are higher with less powder and have no pressure signs, good for you and be happy with that and stop. If you are higher in velocity, per powder charge and you have stopped gaining velocity and start seeing pressure signs, you better consider that enough and start backing down until the pressure signs go away, no matter what the manual says.
As far as your manual saying you should be at 2800fps at 48 gr or what ever it was and you are there at 45gr, watch your chronograph and if you see this parallel between curves start to go away, your gaining pressure and loosing the respective velocity. That is where the velocity curve is going horizontal and the pressure curve is going vertical or staying the same.

That's a dangerous place to be.
Follow what Uncle Nick stated above my post and you will be ok.
 
Not a ballistician but have had a chronograph for 30 some years. In several tests when I do load workups I see the velocity stop going up as much when nearing maximum velocities and pressure signs starting to appear. For example a 1/2 grain increase in powder shows about a 75 feet per second increase from 46 grains up to 50 grains and at 50 grains a 1/2 grain increase in powder only gets 25 fps more. I've seen examples of this in a few different caliber rifles.
 
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LE-28 said:
The problem is when the data as HDWhit stated seems linear, it leads us to believe that if we add another half grain of powder that the velocity will continue to increase the same amount you have been seeing.

Non-linear increase in velocity with charge weight is one of the generally accepted pressure signs, as a wide flat spot or a drop in velocity with increased charge is a sign the chamber is stretching so the powder is burning in a bigger volume. This is why it is often associated with sticky bolt lift. The stretched steel returns to shape, clamping down on the less elastic brass. However, short spans (a few tenths of a grain, typically) of increased charge weight where there is no increase in velocity followed by getting back on the original velocity trend line are more likely due to pressure waves in the barrel and a number of people use them to tune loads. I find there is more to load tuning than that, but it's one observable effect that can be of use.
 
Non-linear increase in velocity with charge weight is one of the generally accepted pressure signs, as a wide flat spot or a drop in velocity with increased charge is a sign the chamber is stretching so the powder is burning in a bigger volume. This is why it is often associated with sticky bolt lift. The stretched steel returns to shape, clamping down on the less elastic brass. However, short spans (a few tenths of a grain, typically) of increased charge weight where there is no increase in velocity followed by getting back on the original velocity trend line are more likely due to pressure waves in the barrel and a number of people use them to tune loads. I find there is more to load tuning than that, but it's one observable effect that can be of use.

Thank you for clarifying that for me. I always wondered what was taking place when I reached that point.
 
To attain maximum velocity for a given bullet, you want to burn as much ideal burn rate powder as possible without exceeding the maximum working pressure, and holding the pressure for as long as possible.
Why does a .30-368 Wby at 62000 psi push 168 gr bullet faster than a .308 Winchester at 62000 psi, all other factors equal? There are many factors, but the main two are:
1. The .30-378 burns more powder releasing more energy.
2. The 62,000 psi is over a much larger area in the WBY. (Think of it like a 1" bore hydraulic cylinder vs a 10" bore hydraulic cylinder both being input 1000psi.)
 
I have never figured out why there are flat spots in the velocity curve but I am glad they are there. A charge with a ES of less than 20 over a three tenths spread is a beautiful thing to find.

You want to see some real velocity changes with different burn rate powders get a 30" barrel and be prepared to start real low when changing powders. H4350 is scary fast in a long barrel. It will also blow primers before you ever get close to Hogdons recommended max. Don't ask me how I know that
 
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