How do you know your case pressure?

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toxic

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There is always data in the reloading tables on pressures and I understand that loads that create more pressures can be more dangerous. I assume that data on pressures is gathered through lab tests.

In another post someone asked about how crimping affects pressures and there are some intelligent sounding answers in the post but I don't understand how they can quantify an increase or decrease in pressure. Is there a calculation that I am missing based on velocity, bullet weight and published pressure data for different powders? Or is this based on a straight velocity increase/decrease vs non-crimped loads?
 
With experience you will be able to tell signs of excessive pressure(but not always!). Speer, Hornady etc. use either copper crusher equipment (CUP measurement) or the use the more modern piezo-electric transducers(psi measurement). Other than that it is an educated guess based on the appearance of the case and primer. There are computer programs that can be pretty accurate too. Quickload comes to mind.
 
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Personally I've always relied on velocity increase and a variety of other factors. So, when testing crimped vs non-crimped way back when, I used a chronograph to measure velocities and was kind of hoping I would see less variation with crimping. One thing I do routinely when starting out with a new load is mike factory loads around the web before and after firing to see what increase in diameter occurs after the round is fired. I don't go beyond this amount of expansion as one way of ensuring my loads are safe (kind of takes advantage of all the good work guys have done in the manufacturing end of the game). I guess from that point of view I'm using web expansion as a measure of pressure too. It was not my idea to do this. I think a chap named Miller first described it, but I can't be sure now. Fred
 
Without the proper equipment it is a wild ass guess.
I've seen guys go well and truely beyond published data and say, 'I see no signs of pressure'.
The problem with 'pressure signs' like flattened primers and the such - once you see them, you are likely to have been beyond maximum for a bit.
I really don't get beating the hell out of guns for an extra 100 - 200 ft/sec.
 
What Pressure?

Miking case head expansion,well, at times is our best and only option,but it is very impressise. There have been a number of writers using stain guages vs case head expansion. Their results were all over the place. If we don't have strain guage installed,they are almost affordable now, velocity is the next best way. This comes with a couple of warnings,too. The reloading manuals don't even agree, they have different ways,test barrels vs sporting barrels.Barrel length is one,test barrels are usually 24" but not always and sporting rifles 22".Then there are slow high friction barrels,( there are no fast barrels). I have used my chrony that way in developing loads for my 338-06 and 264WM. I used suitable powders and with realistic expections.The other thing I use is the radius of the soft shelled CCI primers.
 
The most reliable way that I have seen for deciding if a load is safe is by primer pocket expansion.If you load and fire the same new case five times with a given load,and the primer pocket is still tight,the load is likely safe in your gum.If the primer pocket is loose after five firings,the load is likely too hot for your gun.As for chronographs,they measure velocity not pressure,and the two are not always directly dependent on one another.A rough bore can produce less velocity with the same pressure,than a smooth bore that produces more velocity.
 
downwindtracker2 said:
Miking case head expansion,well, at times is our best and only option,but it is very impressise. There have been a number of writers using stain guages vs case head expansion. Their results were all over the place. If we don't have strain guage installed,they are almost affordable now, velocity is the next best way. This comes with a couple of warnings,too. The reloading manuals don't even agree, they have different ways,test barrels vs sporting barrels.Barrel length is one,test barrels are usually 24" but not always and sporting rifles 22".Then there are slow high friction barrels,( there are no fast barrels). I have used my chrony that way in developing loads for my 338-06 and 264WM. I used suitable powders and with realistic expections.The other thing I use is the radius of the soft shelled CCI primers.
Click to expand...

Yes, I wondered how good measuring around the web would be. But it is like you say, one does what one can. At the time I was reading about this topic, I also found that various ways of measuring pressure, e.g. LUP vs CUP etc., don't always correlate. I wonder if the industry has settled on one particular way of testing for pressure now?
 
stubblejumper said:
The most reliable way that I have seen for deciding if a load is safe is by primer pocket expansion.If you load and fire the same new case five times with a given load,and the primer pocket is still tight,the load is likely safe in your gum.If the primer pocket is loose after five firings,the load is likely too hot for your gun.As for chronographs,they measure velocity not pressure,and the two are not always directly dependent on one another.A rough bore can produce less velocity with the same pressure,than a smooth bore that produces more velocity.
Click to expand...

No argument that expanded primer pockets indicate excessive pressure, but I have always felt that the pressure was way too high if the primer pockets expand. Personally, I would want to stop before that happens. Admittedly it's a matter of memory, but I don't recall primer pocket expansion happening when it wasn't preceeded by other changes, e.g. flattened primers, sticky bolt, excessive noise, web expansion, and so on, that would lead me to stop increasing my load.
 
Primers are the worst way to guess at pressure. Some brands are tougher than others and will not flatten as much, with the same pressure. The primer cratering is also not reliable, because it usually indicates too large a hole for the firing pin.
I too, think primer pocket expansion is the surest, but, it could be much too high a pressure that expanded it, if it was done on one shot. Shooting fairly high pressure a number of times with the same brass will expand it. This is why it is very important to check every time to see if the primer is seating tight enough.
My experience has been that the bolt will stick on a heavy load before one shot will expand the primer pocket. If the bolt gets sticky to open, your load is too high. The reason the bolt is hard to open, is because the case is stuck in the chamber. When the bolt opens, it first turns a bit before the case is broken free from the chamber. This will make a shiny spot on the cartridge face, where the bolt turned. Examine the fired case, to see if it has this shiny spot, in the form of a circular arc, where the bolt face turned. If this mark is present, the load is too high.
One other thing. If we were creating these loads with a fast burning powder that came well short of filling the case, or maybe half filled the case, this operation of working on the top limits of pressure, would be extremely hazardous.
Higher pressure loading should always be done with a powder that about fills the case. Hopefully, comes right to the base of the bullet, or, some slow powders work well with a bit of compression.
 
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Pressure Measurments

The old copper(or lead) units of pressure CUP suffered from the same problems we have when we use case head expansion.In theory,PSI measurements and CUP measurements should correlate very closely,enough have said they don't,to make CUP very suspect. Dr. Ohler ,the experts' expert, strongly says we should forget about the old CUP,and use only PSI.
 
Primer Pockets

I know primer pockets are not reliable for pressure guaging. Case in point: I have a couple of 30-06s,a M-700 and a BSA P-17. In order to tell wich load is for which rifle I use the headstamps,the Remington gets some old lot Norma and the P-17 some new Winchester. Both loads use the same bullet,a 180 Interbond.The load for the M-700 uses the slow burning N-560 at 2740,this is scarcely faster than factory loads. And this velocity been has tested at well below 270 and magnum pressures.These cases only last about five times before they don't fit the shellholder.Butter soft. In developing a load for the P-17, I ran into "oh sh**" pressures and thus the velocities with a low charge weight of H-4350. Of note,H-4350 will spike. Those Winchesters still have nice and tight pockets when I switched to IMR-4350.
 
I know primer pockets are not reliable for pressure guaging. Case in point: I have a couple of 30-06s,a M-700 and a BSA P-17. In order to tell wich load is for which rifle I use the headstamps,the Remington gets some old lot Norma and the P-17 some new Winchester. Both loads use the same bullet,a 180 Interbond.The load for the M-700 uses the slow burning N-560 at 2740,this is scarcely faster than factory loads. And this velocity been has tested at well below 270 and magnum pressures.These cases only last about five times before they don't fit the shellholder.Butter soft. In developing a load for the P-17, I ran into "oh sh**" pressures and thus the velocities with a low charge weight of H-4350. Of note,H-4350 will spike. Those Winchesters still have nice and tight pockets when I switched to IMR-4350.
Click to expand...

If the cases won't fit the shellholder after five firings,the load is too hot for the brass being used.
 
BCFred said:
At the time I was reading about this topic, I also found that various ways of measuring pressure, e.g. LUP vs CUP etc., don't always correlate. I wonder if the industry has settled on one particular way of testing for pressure now?
Click to expand...

LUP (Lead Units of Pressure) is a similar system to CUP (Copper Units of Pressure). They both use a "crusher" in which a small calibrated piece of metal (lead, or copper) is permanently deformed. Measuring the amount of yield (with a micrometer) then indicates (via a table lookup) a measurement of "units of pressure", which are in some sense supposed to be approximately the same as psi (but there are a number of complicating factors that results in LUP != CUP != psi).

LUP is used for lower pressures (typically 7,000-15,000 psi range), and so is commonly used for measuring pressure for shotguns.

CUP is for higher pressures, and is used for handgun and rifle ammo (e.g. 20,000-60,000 CUP).

LUP and CUP are old (100+ year) methods of measuring chamber pressures.

Another newer method uses a piezoelectric pressure sensing transducer. It indicates instantaneous chamber pressure in psi. If you see load data that gives pressure in "psi", it is almost always the case (read the footnotes) that these measurements were done with a piezoelectric transducer.

Another method measures pressure indirectly. A strain gage cemented to the exterior of a chamber will accurately measure the amount that the chamber stretches when the round is fired. By making some measurements (i.d. and o.d.) and some guesses (no residual stress distribution in the steel, and an assumed stiffness of the steel), one can come up with a conversion factor that connects the amount of stretching ("microstrain") to the pressure. But this is an indirect, noncalibrated method. Still, the results can be remarkably useful and insightful, especially if you understand the strengths and weaknesses of this technique.

The industry has settled on using CUP and/or piezoelectric psi for measuring rifle and pistol chamber pressures.

Hobbyests have used strain gages to good effect for the past 15 years or so.

The correlation between bolt sticking, primer pocket swelling, primer cratering, case head expansion, bright ejector marks etc. will vary based on a number of factors. They can be good "relative" indicators, but they are absolutely not "absolute" pressure indicators, for all the reasons listed above, and more (e.g. a small case head, and a small primer pocket, are inherently stronger; so a .223 will take far higher pressures before it starts to indicate or show the same signs as a .308)
 
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