Pressure signs vs velocity

Velocity changes are an indicator of pressure changes. But they are a poor indicator of actual peak pressure.

BBarn said:

Velocity changes are an indicator of pressure changes. But they are a poor indicator of actual peak pressure.

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Going back to the original post, the question is, when I replicate the components in the book, does muzzle velocity indicate pressure? The answer is, it does, and very well.

Further, peak pressure is very highly correlated with the area under the pressure curve. MV is well correlated with both total pressure and peak pressure.

If you have the same length barrel as the book, and are using the same components, and are getting book MV, you are also getting book pressure. I have made many measurements of pressure and velocity over the years, and this holds up very well.

Average pressure correlates best to velocity (in a relative way). Not so with peak pressure. And peak pressure doesn’t necessarily correlate well to average pressure (area under the curve).

denton said:

Going back to the original post, the question is, when I replicate the components in the book, does muzzle velocity indicate pressure? The answer is, it does, and very well.

Further, peak pressure is very highly correlated with the area under the pressure curve. MV is well correlated with both total pressure and peak pressure.

If you have the same length barrel as the book, and are using the same components, and are getting book MV, you are also getting book pressure. I have made many measurements of pressure and velocity over the years, and this holds up very well.

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I havent been reloading near as long a lot of guys here but I dont know if I have ever used all of the listed components let alone had the same firearm as used in the test.

BBarn said:

And peak pressure doesn’t necessarily correlate well to average pressure (area under the curve).

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But it does, at least if you are sticking to the question posed in the OP.

The length of your barrel is fixed. The integral of pressure over distance is the work done to accelerate the bullet.

You get more area under the curve by increasing the peak.

Total area under the curve is very highly correlated with peak pressure.

denton said:

But it does, at least if you are sticking to the question posed in the OP.

The length of your barrel is fixed. The integral of pressure over distance is the work done to accelerate the bullet.

You get more area under the curve by increasing the peak.

Total area under the curve is very highly correlated with peak pressure.

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It’s been 20 years since Calculus III, but I thinks it the double integral of the pressure curve to find the area under the curve. Maybe?

denton said:

You get more area under the curve by increasing the peak.

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This may or may not be correct depending upon a lot of variables. Burn rate of powder, duration of burn, etc...

Ru4real said:

It’s been 20 years since Calculus III, but I thinks it the double integral of the pressure curve to find the area under the curve. Maybe?

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I also seem to remember that a higher peak doesn't always mean more area under the curve.

chris

denton said:

But it does, at least if you are sticking to the question posed in the OP.

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I think we are looking at the situation differently. You've seen good correlation between peak pressure, average pressure, and velocity from a specific set of components in a specific firearm. I believe that.

The OP is using a different firearm with a different barrel than Alliant used, and has no pressure data from his firearm. I don't have enough experience to suggest that matching velocity results from his chronograph to Alliant's published velocity is an accurate way to verify peak pressures reached in his firearm. Perhaps your experience is sufficient to do so.

Erief0g said:

Can we extrapolate that if we reach this speed with our chronograph then we have reached max?

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I think that might be the worst thing about chronographs and published velocities with load data, people trying to achieve some written number.

Say we had the exact bullet listed in the load which it doesn't specify, but for arguments sake, let's assume the exact bullet. We see that listed max for this bullet is 4.7 gr at 1140fps with a four inch barrel at specified oal.

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How about not starting at “max”.

If the book says 4.7 @ 1140 max, how about you start at the beginning and see if the starting load of 3.8 gets you 950 fps, or whatever the starting load is. If your data doesn’t equal the book at the beginning, it would be unreasonable to assume you reach “max” at the book’s velocity.

ballman6711 said:

I also seem to remember that a higher peak doesn't always mean more area under the curve.

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bullseye!

murf

F=ma.

The bullet cannot refuse to accelerate according to the net force applied to it. Think propulsion force minus friction and engraving force and energy diverted to angular momentum of the bullet. Further, the bullet cannot accelerate more than the applied forces dictate. That would violate Conservation of Energy.

The general shape of the pressure curve is fixed. There are no propellants that give a rectangular or a U shaped pressure vs. time or pressure vs. distance curve.

Given those constraints, if you are using the same type of powder and the same bullet mass as the book, muzzle velocity is highly determined by peak pressure.

If you take the book velocity and adjust it to your barrel length, when you reach that velocity you are very close to the peak pressure that the test loads in the book generated. Seating depth, leade, brass thickness, and chamber tightness all influence pressure, it's true. But more or less pressure produces more or less muzzle velocity.

SAAMI does not just specify pressure. SAAMI also specifies velocity.

Sierra's ballsticians have publicly stated that you can increase or decrease charge to get book MV (adjusting for barrel length, of course). John Barsness has been saying that for years. Federal Cartridge adjusts charge to get specified MV. And, yes, my own measurements on several different rifles support the same conclusion.

jmorris said:

I think that might be the worst thing about chronographs and published velocities with load data, people trying to achieve some written number.

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Good luck to those that try. However, that’s not what I use a chrono for. In regard to this thread, if I duplicate what the published load data has, and am under the velocity published, is there some confidence I am also under the published pressure and am operating in a safe range.

BBarn said:

There is no better way to ensure reloading safety than to stay at or below published pressure tested data, with a corresponding absence of high pressure signs.

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I don’t have the capability to measure pressure. I can only measure velocity. With pistol, I’ve observed some high pressure signs (blown case web) and knew something was wrong, but wasn’t willing to test those loads with a chrono to see what the velocity was. The collective advice from this forum is that you’re over pressure limits by the time you actually see pressure signs. I really only want to stay safe!

denton said:

SAAMI does not just specify pressure. SAAMI also specifies velocity.

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https://saami.wpengine.com/wp-conte...FP-and-R-Approved-2015-12-14-Posting-Copy.pdf


“Due to the fact that sporting firearms for general distribution are typically manufactured to dimensional tolerances greater than those specified for test barrels, there should be no expectation that these velocities can be duplicated from any test utilizing firearms.”

Given those constraints, if you are using the same type of powder and the same bullet mass as the book, muzzle velocity is highly determined by peak pressure.

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,Bullet type, burn rate and barrel length, throat, seating depth (jammed/jump).

Estimating pressure from a chronograph reading is indirect at best. Nothing other than direct measurement will give you a certain peak value.

The average pressure behind the bullet during its time in the barrel is proportional to the square of the bullet velocity.

Average pressure is a combination of peak pressure and the much lower muzzle pressure.

Muzzle pressure varies more proportional to powder charge than the more exponential peak pressure. Velocity can reflect a larger change in peak pressure than muzzle pressure, how much more depends on the the curve and what part of it you are in.

lordpaxman said:

The collective advice from this forum is that you’re over pressure limits by the time you actually see pressure signs. I really only want to stay safe!

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But other factors can produce signs of high pressure that are not really high pressure. For example, excessive headspace will produce cratered/flattened/blown out primers.

keep an eye on your ejected brass. if factory ammo ejects brass six feet out and your reloads are ejecting brass 12 feet out, you may be over max pressure.

luck,

murf

denton said:

SAAMI does not just specify pressure. SAAMI also specifies velocity.

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SAAMI lists 'nominal' velocities. Example, 357 max lists speeds for 125 grain bullets from 1220 fps to 1425 fps.

Also, selecting the right powder can significantly change speed. Example, SAAMI lists for the 38 Super 125 grain bullet a nominal speed of 1230 fps. But using N105 or A#7 can push that bullet to 1450 fps and still remain within normal SAAMI pressure limits. https://www.shootingtimes.com/editorial/super-powders-for-the-38-super/99160

murf said:

keep an eye on your ejected brass. if factory ammo ejects brass six feet out and your reloads are ejecting brass 12 feet out, you may be over max pressure.

luck,

murf

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Ejection distance is a function of slide velocity which is a function of slide weight and various springs, not chamber pressure.

fxvr5 said:

Ejection distance is a function of slide velocity which is a function of slide weight and various springs, not chamber pressure.

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then maybe i misunderstood newton's third law of motion. chamber pressure is the force that is acting on the slide. more pressure means more force applied to the slide which means more slide velocity. more slide velocity means a more forceful case ejection. a more forceful case ejection means the case lands farther from the gun.

you can run these steps backward to understand that cases farther from the gun means more chamber pressure.

murf

buck460XVR said:

^^^Yep. What I have found with my handgun reloading, is there is a point where dumping more powder in a case is a diminishing return. When developing loads, there always seem to be a point where adding more powder does not mean significant increases in velocity.

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True. But it does depend on the powder. Some are pretty linear.

denton said:

If you take the book velocity and adjust it to your barrel length, when you reach that velocity you are very close to the peak pressure that the test loads in the book generated. Seating depth, leade, brass thickness, and chamber tightness all influence pressure, it's true. But more or less pressure produces more or less muzzle velocity.

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But consider data such as this:

For the 223, Hornady lists a max load of 24.9gr. of Varget behind the 60gr. Vmax bullet, using Win cases and primer. With that load they report a MV of ~3000fps in a 26" barrel. Hodgdon lists a max Varget load of 27gr. with the Hornady Vmax, with the same brass, and primer. Hodgdon reports a MV of 3159fps with their load.

Using your line of reasoning, there seems to be only two ways to reconcile this:

1) Hornady's data is for multiple bullets and the actual Vmax data is 2.1gr below max.

2) Hornady's data is limited to substantially less pressure than the Hodgdon data.

Are we to assume only one source is correct? And if so, which one?

JohnKSa said:

Linebaugh did some experimentation with straight wall revolver cartridges and found that the traditional reloading pressure signs weren't a great way to gauge pressure in those cartridges.

"Straight cases handle pressure differently than bottle-neck cartridges and often show no excessive pressure signs. We have blown a few guns up here, on purpose, and in all instances upon recovery of the cylinder fragments and case remains, the primer has shown normal pressure. Pressures in these instances have run from 70,000 to over 100,000 psi in our estimation. Do not depend on case pressure signs for danger signs in a sixgun. In most cases the first sign of high pressure you will have, other than excessive recoil and blast, is a bulged cylinder or cracked bolt notch.”

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Bingo! While some of his other writings are somewhat dated, I default to Linebaugh on this subject. So-called pressure signs in revolvers can show up at any pressure level. Especially sticky extraction.

There are also a lot more variables affecting velocity in a revolver that are not present in other firearm types.

243winxb said:

If he is blowing up guns at 70,000 PSI, he needs to use better steel for the cylinder.

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Three points.

A. He's blowing factory guns at 70,000psi to find their breaking point. Factory Ruger Blackhawks in .45Colt blow at 60,000psi. Which is why data is limited to 30,000, a 100% safety margin.

B. Carpenter 465 is extremely challenging to machine. With custom five shots, it's unnecessary anyway.

C. That particular alloy is stainless and was not available when Linebaugh wrote his articles.

murf said:

then maybe i misunderstood newton's third law of motion. chamber pressure is the force that is acting on the slide. more pressure means more force applied to the slide which means more slide velocity. more slide velocity means a more forceful case ejection. a more forceful case ejection means the case lands farther from the gun.

you can run these steps backward to understand that cases farther from the gun means more chamber pressure.

murf

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Pressure and force are not the same. Force is pressure times area in this case. More pressure on the slide (actually force) does mean more velocity for the same recoil spring. Velocity of the slide is a function of the mass of the slide and the stiffness of the recoil spring. Using a stiffer recoil spring reduces the case ejection. Looking at how far the case is ejected is useless for pressure indications.

From this link on Enos, the post written by IDescribe, one of the signs of over pressure, maybe the best in my opinion, are that the velocity increase becomes non-linear with increases in charge.

https://forums.brianenos.com/topic/221357-how-do-you-use-a-chronograph-to-work-up-a-load/