Velocity as pressure sign

I think to say velocity x means I reached my pressure y limit is a dangerous assumption.

Guns do not all shoot to the same velocity. But the pressures can, and often will be nearly the same for the batch of ammo in each gun.

In Speer 15 I believe, they tested 357 handguns. Some guns did 1200 FPS, some less some 1500, with the same ammo. The article was basically a why does my velocity not match yours article. And it shows a wide variation because there are many factors other than pressure alone that determine velocity.

For a rifle, pressure signs can be near the maximum of the gun and ammo combo. But for handguns in most chamberings, pressure signs do not appear anywhere near the pressure limits of gun and ammo combination. Velocity as an indicator, as well as staying within published data is often the only means of determining a safe load in a low pressure round. Recoil, and ejection in automatics, are often instrumental, but not necessarily indicative of pressure.

The slope of the pressure curve is exponential. Humans do not think exponential, and as Al Bartlett said, the greatest shortcoming of the human race is its inability to understand the exponential function.



A longer lecture, is here:



What is true about velocity is that the velocity will always increase as the pressure increases. But velocity will not increase at the same rate as the pressure increase. I do use a chronograph to evaluate velocities, and I can say whenever my loads equal or exceed book values, I am at a maximum load. I find the physical indications of pressure to be inexact and misleading. If I get pierced primers, blown primers, expanded case heads, or sticking cases, these are positive indications of excessive pressure, regardless of what the chronograph is showing. And regardless of whatever other physical indications I observe.

I regularly lubricate my rifle cases so I can see the transition from rounded primers to flat primers. I have found, time and again, that dry cases in dry chambers will show flat primers, at the same conditions that a lubricated fired case shows rounded primers. However, primer roundness is an inexact and crude method of determining pressure. In fact, I am going to claim that all the methods we reloaders have, are inexact, crude, and down right misleading methods of determining chamber pressures. I remember developing loads in my 270 Win FN Deluxe, I was able to push 130's above 3000 fps, no blown, pierced, leaking primers, no sticking extraction. I thought I had found the load baby! Later, after sizing, trimming, I was able to push primers into some of those cases with my fingers! Oppsie, maybe that load is a bit hot!

Also, I am going to say, the more you develop loads, the more you shoot them, the sooner you will be cutting those loads, due to over pressure indications found later.

Have you heard of the 30 exponential steps? If your first step is 1 meter long, and the second is 2 meter, than 4, than 8, just how far will the 30th step take you?

https://knowledgeworks.org/resources/exponential-change/

With pressures increasing exponentially, its that last step that blows everything all the heck.

Now as to why guns blow up. If someone stresses a metal structure, it will break. I thought this was an interesting post, it was about a Ruger Super Blackhawk revolver which had blown its top strap. And this guy was explaining why it was possible for a Ruger SBH to blow in 650 over pressure rounds.


http://castboolits.gunloads.com/showthread.php?150409-Ruger-om-44-convertible&highlight=convertible

Just a few thoughts on this. For Background I am a mechanical engineer with a heavy background in failure and fatigue.

I wonder if I could request a high quality photo of the fracture zone of the cylinder? I am specifically interested in the grain structure of the bolt notches.


I put fort the following.

1) Firearms in general (the type we plebeians can get our mits one) are not designed for infinite fatigue life.

2) The Factors of safety used in firearms design are in line with low end of fatigue requirements (usually less than 10,000 cycles).

3) One of the funny things about fatigue is that each time you push the material past its original design point, you lower its expected life.

4) I am looking at this as an older gun with an unknown number of rounds through it. but based on its age a substantial round count seems likely.

5) When these firearms are designed it is generally preferable for something else to go before the cylinder lets go and takes the top strap. Generally this takes the form of the gun wearing loose or the barrel wearing out. But they are designed to handle X rounds at standard pressures.

6) I see alot of folks calculate the strengths of Rugers, but these calculations are only ever performing an evaluation on a straight static pressure basis. This is wrong when trying to determine if a load is safe.

I attached a couple of marked up figures for your perusal

Click to expand...

Fatigue is real. A structure can go without any warning. You are lucky if you see fatigue signs before the structure fails. One reason why die penetrant tests are used, because the cracks are too small to see without the die.

This was an interesting article : Are your guns tired and stressed?

American Rifleman March 2015

Just today, received a Handloader Magazine. A letter was sent in and an owner of a Third Generation Colt SAA in 45 Colt copied some load data with Titegroup and 250 grain bullets. He stopped shooting at the second shot and could not extract the fired round. When he was able, he found his load bulged the cylinder stop notch. That is the thinnest part of the cylinder. He was lucky the cylinder did not burst as that would have blown the top strap. The gunwriter recommended replacing the Colt cylinder with a 357 Colt cylinder, and have that cylinder reamed to 0.452 chamber throats. Colt seems to have a lot of 0.458" cylinders in 45 Colt, and with a 0.452 bullet, accuracy is poor with 0.458 chamber mouths. The reloader is looking at probably $500 to $700 if not more, to get his ole thumb buster working. Pressures can get out of hand quickly.

Some basics....

Pressure doesn't happen all at once. In a rifle, it ramps up for about.2 to .3 milliseconds, peaks out, and then decays for another .8-1 millisecond or so. What we worry about is the PEAK pressure, because that is the pressure most likely to break things.

The area under the pressure curve and the mass of the bullet, with a couple of finagle factors thrown in, determines how much speed the bullet will acquire while going down the barrel.

That's the basic physics. Now, onward.....

The area under the pressure curve is HIGHLY correlated with the peak pressure. The curve extends from one end of the barrel to the other, or from when the bullet starts to when it exits, depending you how you prefer to do it. But in any event, the two ends of the curve are fixed, or nearly so. The main thing you can do to get more area under the curve is increase the peak pressure.

So it is no surprise that for any particular powder, muzzle velocity is highly correlated with peak pressure. For this to hold, you don't get to fiddle with different kinds of powder.

Since the load data in the book was developed with the powder you're loading, once you reach "book velocity" (correcting for barrel length), you have also reached maximum peak pressure.

I'll keep it simple. Some cartridges don't show pressure signs. If your 45acp is showing you a load a 1000fps load with a 230 you better be pumping the breaks.

Powders that burns to the end of the barrel , produces the highest velocity.

Fast burn rate powders, peak pressure sooner & use less powder. Lower velocity.

When using data from a manual, same components, the velocity should be close to the maximum listed. But the powder used may have a 10% difference in energy, from lot to lot.

Do to this 10% difference in powder lots, velocity recorded, is less valuable. (H4831)

barnfrog said:

Is it impossible for the bullet to reach the published velocity without also reaching the associated published pressure?

Click to expand...

No. It’s possible to reach published velocity without reaching associated published pressure. Barrel resistance, or bullet friction, is the biggest counter force to expanding gasses. By a lot. As one example, you will reach target velocity in every barrel smoother than the equal length test barrel for the same cartridge components as the loading manual.

barnfrog said:

How is it that muzzle velocity can indicate dangerous pressure levels are being approached before the case or primer show the effects of that pressure?

Click to expand...

If you have a tight tolerance chamber and bolt, it’s possible velocity will show first.

Ok, I'm going to take on some of the OP's observations, and answer them with my personal practices. As far as the practice of sneaking up on max, wringing velocity out, hotrodding, and watching for pressure signs... I don't. Especially with bottleneck rifle cartridges.

For the most part, I will take published load data, find a bullet I want to use, a powder I like, throw an 85% charge of max, and shoot it. I will experiment with bullet seating depth, sneak up or down a grain or two of powder, maybe different brands of bullets of the same weight, but that's pretty much it. I am a fan of turning case necks, and keeping brass trimmed. My accuracy is usually above average to excellent. My brass lasts for ever, and copper fouling in the bores is minimal.
I shoot paper, and steel, and do my share of hunting. I am pretty satisfied with the performance of most of my rifles.

One other thing (just to pile on).
Velocity doesn't just "increase with pressure"
It increases with the cube-ROOT of the pressure ratio.

Case-in-point: 30-06 / SMK / IMR4895 / 24" barrel
- Quickload predicts: 2,742fps at 49,728psi
- LabRadar tells me: 2,866fps

Probable pressure is therefore 49,728psi x (2,866/2,742)^3 = 56,784
(... a little goes a long way)

~~~~~~ UncleNick weigh in ~~~~~~~~

The biggest problem is that published manuals tell the reloader information that depends
on the exact components, barrels, bullets, assembly as used at the factory

Trying to match manual-predicted velocity in the field w/o a ballistics program tieing into
chronograph output can get one wrapped around the flagpole real quick.

.

AJC1 said:

I'll keep it simple. Some cartridges don't show pressure signs. If your 45acp is showing you a load a 1000fps load with a 230 you better be pumping the breaks.

Click to expand...

It depends on the powder. Some will get a 230 to 1000 fps within the +P pressure limit, and at least one (Enforcer) will do it within the standard pressure limit.

Before developing a new load I try to figure out what I am trying to accomplish.
If it is my 6.5CM it may be to reach 1,000y before going trans-sonic.
If it’s my M14 load it’s to hit a 600M target with open sights.
If it’s my 300RUM it’s to have a certain amount of kinetic energy at a certain distance

You must also keep in mind that we are barely into spring temperatures so any load development done now must leave some room for the coming summer rise in outdoor temperature.

Velocity is not a reliable indicator of peak pressures in a given firearm.

This is a problem that iterates in the black powder revolver community. Because the pressure with smokeless loads using 250 grs bullets at less than 800 fps is well within the 14kpsi of the 45 Colt SAAMI max pressure, it is often advised to shoot “Cowboy” loads or “keep velocity below 800 fps” as guidance on what smokeless loads are safe to shoot in conversion cylinders in black powder revolvers. And it’s pretty much true. Smokeless loads delivering sub 800 fps loads with 250 grs bullets are almost certainly under the 14kpsi max pressure and there should be no problem accordingly.

Similarly, any load using one othe well established powders for 308 Win delivering150 grs bullets at 2400 fps from an 18” barrel should be well within the max pressures for 308 Win. But, if one tried to use Titegroup with 150 grs bullet in said rifle and used 2400 fps as your measure of safety, bad things would happen.

And while the experienced hand loader can guess loads for certain cartridges and components, ie I can visually load a 30-06 case to “nearly touching” the base of a cannelure equipped 165 grs bullet with H4350 and be pretty sure I won’t be exceeding max pressure for the 30-06, that doesn’t make it good/safe/sensible practice.

The best way to ensure that pressure is below the maximum levels for the cartridge in question in a specific firearm is to use published data from a reliable source and work the load up from below mid point of the data searching for the optimum accuracy/velocity balance while looking at the conventional pressure signs (primers, sticky bolt lift etc) because pressures can differ from firearm to firearm with the same exact load. Every other method represents an increased (to varying degrees) level of risk. Using velocity as one’s method of extrapolating or guessing pressure represents, in my view, an unecessarily high degree of risk.

Not really. I have a 6.5 Creedmoor load that if I go up 1gr higher with N555 I get slight over pressure signs and maybe a 10fps boost in velocity.

MEHavey said:

One other thing (just to pile on).
Velocity doesn't just "increase with pressure"
It increases with the cube-ROOT of the pressure ratio.

Case-in-point: 30-06 / IMR4895 / 24" barrel
- Quickload predicts: 2,742fps at 49,728psi
- LabRadar tells me: 2,866fps

Probable pressure is therefore 49,728psi x (2,866/2,742)^3 = 56,784
(... a little goes a long way)

Click to expand...

That is a handy formula. Interesting too that I was just reading your posts about this same subject on another forum from 2015.

I feel like velocity data is just one tool in our toolbag. It isn't necessarily the gold standard test but it's a useful tool never-the-less. I shot my poor chronograph a couple of weeks ago and I'm not comfortable not having a replacement. So I'll be spending $380 today to replace it with a magnetospeed because that's how valuable I think velocity information is. For me, $380 is still quite a chunk of change for one tool but it's a handy tool. With the formula that MEHavey shared, it's value should be apparent.

Using velocity isn't perfect but is the best tool most of us have.

A real world story. Loading for my 300 WSM I was using a max load of H4350 and getting almost exactly the predicted speeds with 180 gr bullets (2950 fps). I decided to try Reloder 17. Book said 66 gr of powder would give me 3082 fps. But 64.5 gr of powder showed a bit over 3100 fps on my Chronometer. I backed off to 64 gr (2 gr below listed book max) and never loaded or fired any loads with more than that. But at 64 gr I was just barely faster than my H4350 load and with less accuracy. I decided that for me the slower H4350 load was the best choice.

With that rifle, with that powder and bullet around 64 gr was a max load, not 66 gr. Without my chronograph I could have easily loaded and fired 65 or even 66 gr and still not gotten any visible pressure signs. But would have been overloaded.

The opposite can happen, and this is where someone could get into trouble. Suppose a book load says 48 gr of powder X is supposed to give you 2900 fps. But at 48 gr you're only getting 2800 fps. It would be tempting to say that with THAT rifle it would be safe to go over 48 gr of powder until you reach 2900 fps. And that MIGHT be OK. But I strongly advise against it. There are other issues that might come into play. And if that ammo got into another rifle, it could be dangerous.

When I'm working up a new load, I want to know what velocity to expect with a max load. I won't exceed the max velocity even if I'm below the max charge weight. And I won't exceed the max charge weight even if I'm getting less speed than expected. Without a chronograph you're driving blind about what speed you're getting. And if you're waiting until you see pressure signs, you're already way over max before those show up.

fxvr5 said:

It depends on the powder. Some will get a 230 to 1000 fps within the +P pressure limit, and at least one (Enforcer) will do it within the standard pressure limit.

Click to expand...

So by reading my post you would concur if you had a 1000' fps you would keep adding powder. Or you would evaluate the condition as a probable stopping point. Setting reasonable trip wires for indications is the goal of this post.

JeffG said:

Ok, I'm going to take on some of the OP's observations, and answer them with my personal practices. As far as the practice of sneaking up on max, wringing velocity out, hotrodding, and watching for pressure signs... I don't. Especially with bottleneck rifle cartridges.

For the most part, I will take published load data, find a bullet I want to use, a powder I like, throw an 85% charge of max, and shoot it. I will experiment with bullet seating depth, sneak up or down a grain or two of powder, maybe different brands of bullets of the same weight, but that's pretty much it. I am a fan of turning case necks, and keeping brass trimmed. My accuracy is usually above average to excellent. My brass lasts for ever, and copper fouling in the bores is minimal.
I shoot paper, and steel, and do my share of hunting. I am pretty satisfied with the performance of most of my rifles.

Click to expand...

I'm with you on this. I've never considered velocity an indicator of anything except how fast the bullet's going when (okay, APPROXIMATELY when) it leaves the barrel. I kind of sort of use that to tell me how close or far away I am to the moment, inertia, energy and range I'm looking for; but, I don't even try to interpolate that to mean I'm in a certain pressure range inside the chamber.
The p in p=mv is momentum. The p in p=F/A is pressure. They're not the same value.

This is a plot of muzzle velocity as a function of peak chamber pressure for an 8X57 rifle, loaded with various charges of Varget. I have done many such plots over the years, always with a linear result, with one important exception.

The exception is that with some powders and cartridges, you can reach a point where MV does not advance with more powder. Instead, MV becomes more erratic. My most memorable case of this was with AA2520 in a 308 Win. It was memorable because it was the first time I had seen this effect. A magnum primer is a help in this situation.

So, if you want to use MV as an indicator of peak pressure, you can, so long as you are operating below that plateau. MV and peak pressure are highly correlated, 99% in this case.

Chamber tightness, leade, neck tension, engraving force, friction, etc. all affect volume as a function of time, which in turn is a driver of pressure. Pressure as a function of time drives MV. In a firearm, there is no source of energy to drive a bullet other than gas pressure. There is no magic, only net force vs. time.

If you are using the powder and bullet in a load published by a reputable source, when you reach "book" MV, you have reached "book" peak pressure.

Never one to let real data get in the way of theory () I ran QuickLoad
looking at predicted Pressure/Velocity relationships

Lo & behold for 4895, 3rd-Order polynomial was literally perfect fit (R2=1)

Thinking it was as much a factor of increasing burn efficiency w/ a mid-burn powder
(only 93% at start), I also ran fast 3031 (effectively 100% start to finish)

Still 3rd-Order Polynomial.
`Tis a puzzlement.....

Not too much of a puzzle.

If you look at the equations, the coefficients of your X^2 terms are negative, meaning that more pressure yields lower MV. That is contrary to the physics, so something isn't quite right.

What is probably going on is multicolinearity. That is, your X, X^2, and X^3 terms all occupy nearly the same space. That breaks the regression tool.

The usual practice is to start with a linear term, and then add more terms only if they improve R^2 by at least 5-10.

You have ideal, noise free data. With that, it is clear that there is a little curvature, but not much.

The usual practice is to start with a linear term, and then add more terms only if they improve R^2 by at least 5-10

Click to expand...

... and so I did...
Coming up from the bottom, 3rd degree always hit that magic "1"
What is clear is that the equation(s) are a perfect fit

What's always lacking in these threads is a sampling of multiple rifles of various manufacturers with the same chambering along with data showing that all those rifles produce the same velocities at the same pressure. Such data would confirm that particular rifle variations are of little significance. Without such data, it's just posturing to say the the chronograph tells the story and the rifle doesn't matter.

A cautious loader can use his chronograph to get his load to the predicted velocity. For example, if he is using a .30-'06/180, he can expect 2700 as a reasonable target, and if he gets there with a published load, he can be almost certain he is within established pressure. (This is how I do it.)

A slightly more adventurous fellow can keep going to the maximum published load, enjoy somewhat increased velocity, and still be relatively assured his load his safe, assuming he does not see any pressure signs.

And an unfortunate fellow may find that he cannot get to his predicted velocity with any published load. Some people will continue past published loads in order to achieve that velocity, and while that is a little too bold for me, it probably is not unreasonable. If the top listed load only gets you to 2640 fps with the same length barrel, and 2% more powder gets you to 2700 without pressure signs, you likely are just fine.

On the flip side, if I load up some cartridges and discover I am getting 2900 fps out of them, that's a real warning: it's pretty unlikely that my gun is still operating within SAAMI limits, even if I have no other pressure signs, and I need to step back and do some real thinking about the situation.

That, to me, is the way to use a chronograph as a pressure tester. The only real way to measure pressure is to measure pressure - but if you don't have access to that equipment, a chronograph is far more reliable than things like bolt lift and primer condition.

<edit> In a healthy gun, the only thing the traditional "pressure signs" tell you is that you're over pressure - and likely way over pressure. Backing down until those signs disappear doesn't necessarily mean pressures are either within spec or safe. And in a gun with specific issues, it is possible to have "pressure signs" and still have a load that is within spec. Again, you can't be sure without actually testing the pressure, but a chronograph takes a good chunk of guesswork out of it.