Velocity and pressure

M

MightyMO1911

New member
I know there is myriad reasons why velocities don't always add up to what is published and frankly, if I'm off a little bit I really don't care. But I encountered a large variance this weekend and it raised a question in my mind.

I loaded some 38 special with a 125 grain Sierra jsp and BE86 powder. I do not remember the charge weights so I won't publish them, here. But advertised velocity was 1078 I believe and we barely breached 800.

So my question is this. .. Well now that I'm typing it I'm not sure what my question is. Lol. Ok. So are pressure and velocity related? Would it be a fair assumption to say the pressure is also below published?

Don't read into this. I have no intention of loading beyond published data. I'm simply going to find a different powder. So this really just an academic question.
 
Higher pressures, are going to result in higher velocities.

I wouldn't assume anything though in the world of reloading. That's a good way to get injured.

I recently completed a test I was doing on .308 Win brass, 9 different headstamps. Capacities ranges from 55gr down to 53.5 grains. I started with the ones with the most capacity. As I was shooting the test cases, velocity gradually increased. The cases with the least amount of capacity, had the highest velocities. This is due to less capacity, same amount of powder, higher pressures.
 
It depends.

Published velocity is always with a particular barrel length. If your barrel length is shorter, that cold easily account for all the difference depending on how much shorter it is, and have nothing to do with pressure.

Consumer grade chronographs are not always precise. Especially if you are too close to a sky screen, you can pretty easily false-trigger a sky screen and that can lead to either fast or slow readings, depending on the circumstances. A usual recommendation is to take a .22 rimfire rifle with barrel at least 18" long and fire a few match grade rounds from it. The expansion ratio for .22 rimfire is so large it guarantees exhausting the accelerating force from the powder gases, so it generally comes out within 50 fps of the claimed velocity on the box, making it a good way to check the chronograph mechanism. This test has to be repeated when lighting conditions change as some chronographs are very sensitive to them. One afternoon I had my very trustworthy Oehler 35P set up with 4 foot spacing, while my dad was using a chrony with one foot spacing on the adjacent firing point. His .308 loads seemed to produce too much velocity, so we ran a few over the Oehler, which read 200 fps lower, and more in line with book value.

There are two pressure numbers of interest in firing a cartridge in a gun. One is the peak pressure, which is the number you normally see published. It is important for safety, obviously, and wear and tear on the gun.

The other is average pressure over the length of barrel during the bullet's trip down it. After making a small friction loss allowance (variously estimated at from 3% to 7%, depending on the authority), you multiply that average pressure by the length of bullet base travel in the bore from starting inside the case until the base clears the muzzle. Then you divide it by the bore cross-section area. That gives you the work done moving the bullet to the muzzle. The unit of work is the ft-lb, same as a unit of kinetic energy, and is equal to the kinetic energy of the bullet at the muzzle. The kinetic energy is proportional to the square of velocity, hence, for a given barrel length, the average pressure in the barrel is proportional to the square of velocity at the muzzle.

So, to answer your question, the average pressure affects the square of velocity, but the peak pressure alone does not predict it.
 
I thought I would try to add something to this thread, but Unclenick has all the bases covered.

As he wrote, check the calibration of your chronograph.
 
My only question is, in your loading manual, did your 38 also match the testing barrel length as well? There are just a bunch of variables that play into getting "exact" FPS as published.
 
Getting the exact FPS listed is serendipity. Close is normal, but 100fps up or down with equal barrel lengths is not so rare as you might think.

Even if you could match all your components to the exact same lot#s used in testing, you aren't shooting the same gun!

All the myriad tolerances line up in different ways and can produce results at either end of the bell curve, almost as likely as in the middle.

Even with absolutely equal pressures, some gun & ammo combinations are faster than others.

My experience with chronographs showed me that my results were always within the variance found shooting different guns. Published data is a guideline, not a law.
 
I hardly ever compare my chronograph results with published results. I can't even remember the last time I did so. My results are my results; their results are their results.
 
First off... how much of a crimp did you put on the .38 reloads ? Some revolver powders "need" a good crimp to help the powder burn best.

What barrel length is your gun ? What primer used ?

And ... ahem.... I have found over the decades, Manufacturers "tend" to hype their velocities for the lastest / greatest powder.... cough, cough....

Also, you might find this interesting.....This follows directly with 44 AMP's train of thought. http://www.leverguns.com/articles/ballisticians.htm

Note the dramatic differences in alike 6" barreled revolvers.... including the same brand ones.
 
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You bring up an interesting point concerning crimp. I had a good roll crimp on them but more is always possible. The revolver used was a Ruger gp100 with a 6" bbl.
 
Velocity is proportional to pressure BUT pressure is a curve with a peak!

You can get much higher velocity with a slower powder that ramps up to a safe peak, and maintains that peak for the entire duration of bullet until exit as opposed to a fast powder that peaks to an unsafe level and declines quickly.

If powder is too slow for the round, it will still be ramping up to peak pressure when the bullet leaves.

All you need to do is review load data for your 38 cartridge. You will see that +P (high pressure) loads with fast powders hit their peak pressure threshhold without achieving near the velocity of slower powders.
 
Read the reference chapters of your manual(partial to the Lyman book myself.). Should be something there about pressures and how they're determined. Pretty good read too.
In any case, published pressure numbers are averages of pressures delivered in a pressure gun. So are velocities.
 
I have an interesting scenario that I just encountered that will make you think a little about powder and pressure.

I was loading for a 7mm rem mag and the max load in my load book was 58.6 grains of H4831. When I shot the ammo at 57 grains my cases were dimpled, split, or completing severed near the base. Primers were heavily cratered with blow by and the recoil was massive. So I reduced the load and nothing changed. After several years of trying to figure out what was wrong I noticed that on the side of the powder container the minimum charge was 60 grains for the same bullet weight and after some research found that 67 to 69 grains was a max load. Significantly different than 58 for a max load. So I loaded up some ammo at 65 grains and wouldn't you know it shot nice but not nearly at the velocity it should have so I continued to work up and am now at 70 grains and its shooting awesome. Accuracy is phenomenal as is the recoil which feels much softer. The point is that the powder was detonating a the very low charge that was published and also not expanding the case enough to seal the chamber and that was causing the cases to crush or implode. So now that I am 12 grains of powder over what the load book says my rifle is shooting perfect and very much safer than before. Pressures are lower and velocity is up.
 
I was loading for a 7mm rem mag and the max load in my load book was 58.6 grains of H4831.
Click to expand...

Which begs the question, WHAT book did you use???
(and the secondary question, are you certain you are on the right page???)

58gr H4831 is a max load, in the first book I picked up, for a .284 Winchester!

ALL the reloading books are just guidelines. Don't get your data from a single source and take it as gospel. It ain't.
 
They are related but are certainly not linear.

Higher pressure also will not always equal more speed. At some point your bullet likely wont even exit the barrel as parts of the revolver let go.
 
Any consideration about the barrel / cylinder gap on the revolver?
Perhaps the published velocities are from a test barrel with no gap at all.
 
No. The SAAMI standard test guns for revolver cartridges have a 0.008" (IIRC) gap where a barrel/cylinder gap would be, and the barrel lengths are determined from that point forward. You can download the SAAMI test standard for handgun ammunition and in the last third or so are the test gun drawings by cartridge.


Madgunner,

It's probably not actual detonation you saw, as that shatters steel and disassembles the gun, but pressure does become erratic if you have too much space in a case. The usual explanation is that the powder lies so the primer flame can ignite a larger surface area of it than it does when the powder is more uniformly positioned in front of the flash hole. The result is you get the performance of a powder with a faster burn rate.

There is some data showing this from the old mid-60's study by Dr. Lloyd Brownell.
 
The other is average pressure over the length of barrel during the bullet's trip down it. After making a small friction loss allowance (variously estimated at from 3% to 7%, depending on the authority), you multiply that average pressure by the length of bullet base travel in the bore from starting inside the case until the base clears the muzzle. Then you divide it by the bore cross-section area. That gives you the work done moving the bullet to the muzzle.
Click to expand...

Just a note: work done by the expanding gas is W = pV, where p is pressure and V is expansion volume. Since V = Ah (A is the bore cross-section area, h is the length of the bullet travel), one should calculate the work as W = pAh, that is, multiply the pressure, area and travel length with each other.
 
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