Pressure signs vs velocity

If you reach book velocity with the same length barrel as the book, you are at book pressure.

Erief0g said:

different length barrrel. same powder and same velocity. pressure the same?

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No, not necessarily. Pressure signs, such as a flattened primer or primer flow, are not very good indicators of peak chamber pressure because lots of thing can cause those 'signs' when the pressure is ok.

The recommended method for watching pressure is doing case head measurements on new cases. Hornady used to have a section on this in the manual, it might be on their website.

Erief0g said:

Can we extrapolate that if we reach this speed with our chronograph then we have reached max? This would apply even though changing the barrel length or possibly the cartridge oal?

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No. Barrel length has nothing to do with peak chamber pressure. Chamber pressure peaks, in handgun ammo, when the bullet has moved 1/2" or so, depending on the load. In general, longer barrels mean higher velocity.

OAL will change pressure. The deeper the bullet seated, the greater the pressure (handgun ammo).

different length barrrel. same powder and same velocity. pressure the same?

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Nope.

Usually when I work up a load I try to find the bullet manufacturers data. Doesn't mean I have the same firearm but it gives start and max load data.

That said, my barrel length is usually longer than the listed data was developed in, or sometimes shorter if they used a universal receiver. Given that I might add or subtract 50-75fps or more for MY max velocity given the difference of barrel length. Usually ill determine that while firing start loads and comparing my data with theirs.

If their barrel is a 4" and mine is 7.5" or vice-versa I might see a hundred fps difference, non the less when I get to or get in the ball park of their max I usually hold up. Depending upon what caliber and bullet it is, it could have been designed for a specific range of impact velocities or pressures. No sense pushing them beyond what they were designed for.

The above might mainly pertain to revolvers, but still applies to pistols as well. As mentioned already however the seating depth is a more critical factor with pistols due to more of the loads for them running the faster powders. Sometimes only a few thousands difference can spike pressures in the higher end loads, and even in some specific calibers.

Tagging in since I’m interested in the subject.

Erief0g said:

Example would be the snip pic from Alliant load data. 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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Alliant doesn’t list pressure that I know. Some do, like Hodgdon. Maybe we could ask: “if I have the exact load listed, powder, primer, bullet, COL, AND I achieve the velocity stated for that load, and it lists a pressure, does that mean that’s the pressure generated in my gun?”
I wouldn’t assume Alliant develops their max load data based on max SAAMI pressure.

Variations in firearm barrels (even of the same length) and chambers can significantly affect the relationship between peak pressure and velocity, even if the same reloading components are used. So assuming that the same peak pressure is reached when the velocities match between two different firearms is presumptuous.

My experience is with rifle cartridges, but the same rules apply. Traditional pressure signs don't show up until you are WAY over pressure. Using them as a guide is like driving a car with no speedometer, only a warning light that comes on when you reach 100 mph.

A chronograph is like having a speedometer in your car. When the bullets velocity starts getting close to max speed, you are approaching max pressure.

And as stated above the individual gun matters. I've had rifle loads exceed max speed at 58 gr when the loading manual said 60 gr was a max load. In reality 60 gr was over pressure in MY rifle. It may have been fine in YOUR rifle.

I'm not a huge fan of pressure signs myself, don't like to see them.

I've either cast or swaged a bunch of bullets over the years that there's no data for. At the end of the day I've always looked at case volume after the bullet is seated.




I pay more attention to the case mouth in pistol & revolver calibers. More pointed the amount of suet on the cases, nothing better than a starting load of unique. I like to see the black smudges/suet on the sides of the case mouths with starting loads and then watch them go away as the load increases.

If anything I look for and want under pressure signs the black suet on the cases don't lie.

denton said:

If you reach book velocity with the same length barrel as the book, you are at book pressure.

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No.

The difference in cylinder/barrel gap on a 44 revolver makes a difference in velocities. The groove diameter also.

BBarn said:

Variations in firearm barrels (even of the same length) and chambers can significantly affect the relationship between peak pressure and velocity, even if the same reloading components are used. So assuming that the same peak pressure is reached when the velocities match between two different firearms is presumptuous.

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But of course. I get different velocities from the same load in different firearms. Specifically 9mm in similar barrel lengths. There’ll always be differences due to the manufacturing processes. The velocity differences aren’t a factor of two or ten though. Herr Heisenberg doesn’t let us measure exactly, but with the same components and velocity I’d need to be close to the pressure stated. Otherwise, published data isn’t keeping us safe.

Erief0g said:

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

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No, not very precisely.

denton said:

If you reach book velocity with the same length barrel as the book, you are at book pressure.

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No, not really. You're leaving out too many variables.

A few easy observation:
- a longer throat permits higher charge weight, and consequent velocity, at iso pressure. See Weatherby's extended throats, and the dramatic overpressure generated by seating near the rifling in a Weatherby cartridge.
- a tighter bore (vs looser) will generate more pressure at iso charge, and higher velocity due to more efficient combustion.

Just these two variables (among others) have a significant impact on final velocity out the muzzle, and are enough to shift pressure sign between rifles by 10% charge in my limited experience.

So, the picture perfect proof barrel (SAAMI Min, for worst case) in the pressure gun that the manual data was collected from could be easily 10% different than your gun, maybe more.

This is why we work up! I have a rifle an apparently in spec chamber and throat that shows pressure signs at Start charge for some powders, while his buddy in the same cartridge can reach Max charge easily.

This is also why we watch primer and brass flow, even through they don't perfectly correlate to pressure. In modern arms, the first failure mode is not a kaboom, it's a blown primer or loose pockets. Keeping well shy of the first failure mode will protect you from the second, despite not knowing what the pressure is.

forrest r said:

I'm not a huge fan of pressure signs myself, don't like to see them.

I've either cast or swaged a bunch of bullets over the years that there's no data for. At the end of the day I've always looked at case volume after the bullet is seated.
View attachment 969394

View attachment 969395

I pay more attention to the case mouth in pistol & revolver calibers. More pointed the amount of suet on the cases, nothing better than a starting load of unique. I like to see the black smudges/suet on the sides of the case mouths with starting loads and then watch them go away as the load increases.

If anything I look for and want under pressure signs the black suet on the cases don't lie.

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I have Hornady's manual from the 70s. They gave the seating depth instead of the COL. They was basically giving the case capacity after seating. I wish more of that info was available nowadays, with so much bullet variety out there with no publication.

I do find JBM- Ballistics site useful at times, to compare bullet lengths.
http://www.jbmballistics.com/ballistics/lengths/lengths.shtml

JohnKSa said:

Linebaugh

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If he is blowing up guns at 70,000 PSI, he needs to use better steel for the cylinder.
Ruger - https://www.carpentertechnology.com/blog/most-powerful-revolvers-get-lift-from-aerospace-alloys

Ruger made a cylinder from Custom 465 stainless, then fired 50 proof rounds from each of six chambers (total 300 rounds) at 92,000 psi. This is about 50% higher stress than that produced under actual firing conditions. The designers repeated this firing cycle numerous times and noted that the cylinder successfully withstood all of the proof firing without a scratch or sign of defect anywhere.

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View attachment 969437

Take note of SAAMI Proof Load Pressures. If these pressure blow a gun up, the gun was defective in the first place.
Proof load brass is scrap after firing. View attachment 969441

edwardware said:

No, not very precisely.

No, not really. You're leaving out too many variables.

A few easy observation:
- a longer throat permits higher charge weight, and consequent velocity, at iso pressure. See Weatherby's extended throats, and the dramatic overpressure generated by seating near the rifling in a Weatherby cartridge.
- a tighter bore (vs looser) will generate more pressure at iso charge, and higher velocity due to more efficient combustion.

Just these two variables (among others) have a significant impact on final velocity out the muzzle, and are enough to shift pressure sign between rifles by 10% charge in my limited experience.

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Tighter bores generate more pressure because of the fundamental gas laws relating pressure and volume. Smaller volume, more pressure. More pressure, more velocity.

Cartridge length does influence pressure. Seating very deep or at the lands creates more pressure. And that in turn creates more velocity.

Back in the early 1960s , i get to the range. There are 3 guys , shooting 3 guns. All shooting Ruger Blackhawks, 45 Colts over a chronograph. Note: old chronographs used card board "screens" . The card board had a copper wire embedded. The bullet had to cut the wire on both screens every time, to record velocity.
They all were loading Unique & heavy cast bullets. As the powder charges were increased, case extraction started to become harder.
As loads kept getting hotter, the ejector could no longer push fired brass out of the cylinder, 1 at a time. They solved extraction by removing the cylinder from the frame. Place the cylinder on wood block & hammered the brass out of the cylinder with a steel rod. They came prepaired to do this.

Not recommended to do. But an example that the extraction effort will increase as pressures go up. With my 357 & 44 mags, i can see harder case extraction, before needing to pound brass out of cylinders. Primers show a difference from starting loads to maximum also. IMO.

On auto loaders like AR15 223/5.56 brass will stick in the chamber. The extractor will damage the case rim trying to extract the brass.

Load for accuracy, not velocity. No need or reason to load above published maximums.

.

243winxb said:

Load for accuracy, not velocity. No need or reason to load above published maximums.

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^^^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. Sometimes, one actually starts to see a decrease. This is staying within published recipes. Thus, why add more powder if there is no advantage?

Thinking you need to obtain max published velocities from your firearm, can also be a futile pursuit. Using a stock 4" revolver with a cylinder gap and trying to match velocities obtained by a 10" test barrel just ain't gonna happen.

lordpaxman said:

Herr Heisenberg doesn’t let us measure exactly, but with the same components and velocity I’d need to be close to the pressure stated. Otherwise, published data isn’t keeping us safe.

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Published data isn't based on velocities achieved, it's based on peak pressure reached. Assuming you are at or below measured test velocity does not mean you are within SAAMI peak pressure limits. 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.

BBarn said:

Variations in firearm barrels (even of the same length) and chambers can significantly affect the relationship between peak pressure and velocity, even if the same reloading components are used. So assuming that the same peak pressure is reached when the velocities match between two different firearms is presumptuous.

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Yes, very true. However velocity remains the best of all chamber pressure indicators.

And, as with ALL the items listed in the reloading manual recipe, they are simply a report of the results gained at the lab. They are not a template for exact results.