Practical Chamber Pressure Measurement

Why are you not crimping? If i had problems with bullets creeping deeper. I would pull them and seat and crimp.

The TC is not designed to hold the bullet , neck tension does. If you need more you to expand less after sizing. Depending on brass, you may have to change bullet size or brass mfg.

CoalCrackerAl said:

Why are you not crimping? If i had problems with bullets creeping deeper. I would pull them and seat and crimp.

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"Metallic cartridge reloading" by Mcphearson discusses pressure measurement. Basically you use a .0001 micrometer & measure expansion of the case head.

I don't "worry" about chamber pressure when using manufacturer data. I do focus on following protocols about properly resizing, minimal flare to seat the bullet, and proper crimp when necessary.
On semi auto I test until I'm sure I don't get set back and roll on.
When I wonder off the reservation. I worry about pressure. Like using titewad as a cast bullet powder. A mess up will cause severe damage. But my reward was extremely accurate powder puff rifle ammo.

Spare Parts said:

@LiveLife has posted good info about OAL, especially in uncrimped pistol rounds that "walk" themselves into deeper seating, leading to pressure spikes.

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You are misquoting me.

My myth busting thread on bullet setback was to show certain combination of bullet type/profile, OAL/seating depth, bullet diameter and case wall thickness can produce less/zero bullet setback from sufficient neck tension. This was not done to prevent pressure spike rather to improve reloading variable consistency to pursue more consistent chamber pressures ultimately to improve group size/accuracy - https://www.thehighroad.org/index.p...neck-tension-and-bullet-setback.830072/page-4

CoalCrackerAl said:

Why are you not crimping? If i had problems with bullets creeping deeper. I would pull them and seat and crimp.

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As well documented on multiple threads, increasing taper crimp amount will not prevent bullet setback rather decrease neck tension from sizing bullet diameter smaller and brass spring back after crimping.

Taper crimp is applied at case mouth where case wall is thinnest and greater neck tension is produced further down the case neck around bottom of bullet base where case wall is thicker. To improve neck tension so bullet setback is reduced/eliminated, you need to seat bullet base deeper, use thicker case wall brass and/or use larger diameter bullet.

Spare Parts said:

A chronograph is a good proxy, assuming muzzle velocity predicts peak pressure. However, I have a couple of barrels that always seem to exceed the book spec.

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Book spec or published load data were often tested using "universal barrel fixtures" and depending on groove diameter, leade length to start of rifling and barrel length, will often produce different velocities than our pistol barrels with different groove diameter, leade length and barrel length - https://www.thehighroad.org/index.p...let-max-working-oal-col-for-reference.848462/

Spare Parts said:

I have a few LE/mil clones (HST and Gold Dot in 9mm) that are reeeely close to the edge. Should I lock them with sealant?

If you have data, enlighten me.

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DISCLAIMER: If you want to load "duplicate" factory premium JHP rounds with advertised velocities, I recommend use of new or known once-fired brass. If you are using mixed range brass with unknown number of reloading and brass history, I suggest you use mid-to-high range load data instead of max charge.

I have some data for you to consider.

For decades, I have tried to load "duplicate" factory premium JHP for practice and after switching from HydraShok/Black Talon/Ranger T to Speer Gold Dot and Remington Golden Saber (Remington ammunition now owned by Federal/Speer/CCI/Alliant - Vista Outdoor) based on higher velocities and penetration/expansion test performance, I bought bulk Gold Dot/Golden Saber bullets and used WSF for higher velocities it produced compared to other powders (Besides, WSF was one of my match load powders and produced accurate loads).

When Alliant released BE-86 with "rumors" that it is the same powder that's been used in factory premium JHP ammunition for decades (I am thinking Speer Gold Dot/Federal HST, etc. ), I switched from WSF due to higher velocities and greater accuracy achieved.

If you look at Speer load data, it is sorted by highest velocities to lowest and BE-86 tends to be at the top of the listing.


Load data for 9mm 124 gr Speer Gold Dot HP - https://reloadingdata.speer.com/downloads/speer/reloading-pdfs/handgun/9mm_Luger__124_rev1.pdf

• 9mm 124 gr Speer Gold Dot HP BE 86 OAL 1.120" Start 5.4 gr (1124 fps) - Max 6.0 gr (1199 fps)

Speer advertises 1150 fps for 124 gr GDHP LE - https://www.luckygunner.com/9mm-124-grain-jhp-speer-gold-dot-50-rounds


Load data for 9mm 147 gr Speer Gold Dot HP - https://reloadingdata.speer.com/downloads/speer/reloading-pdfs/handgun/9mm_Luger__147_rev1.pdf

• 9mm 147 gr Speer Gold Dot HP BE 86 OAL 1.130" Start 4.5 gr (936 fps) - Max 5.1 gr (1027 fps)

Speer advertises 990 fps for 147 gr GDHP LE - https://www.luckygunner.com/9mm-147-grain-jhp-speer-gold-dot-le-50-rounds

The only way to know pressure with certainty is to measure it. Case head measurement techniques can be used to estimate pressure. Chronograph measurement techniques can be used for pressure estimates if you have corresponding pressure data from the same gun using the same component combination.

Even the manufacturers of consumer pressure measuring devices caution users to not exceed published reloading data.

A couple of observations,

Firearm chamber pressure measurement isn’t an exact science. Manufacturers claim a variance of 3-5% under exact same conditions.

Velocity is a poor proxy for max pressure. But it’s the best most shooters have. Just keep in mind max peak pressure can be much higher than average max pressure.

Interpolation of pressures IS NOT valid, because burn rate is heavily influenced by flame to propellant boundary pressure.

More than you ever wanted to know about propellants:
https://web.stanford.edu/~cantwell/...tion of Solid Propellants RTO-EN-023 2002.pdf

In my estimation the only semi reliable way to get an idea of the pressures we're creating is by measuring velocities and noticing loosening primer pockets after 2 or 3 reloads. Measuring for expanding case heads supposedly works if you have a very good micrometer. All the rest is voodoo IMO.

There is a commercial product that can do what the OP wants. It's called Pressure Trace (https://www.shootingsoftware.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=RSI&Category_Code=PT) It does require a minor exterior modification to your firearm to mount a strain gauge over the chamber of the firearm. Best practice also requires the use of Standard Ammunition that is loaded to known peak pressures to calibrate the strain gauge to your rifle for the most accuracy before you test unknown ammunition you wish to know the pressure curve of. That said you can do it without calibration in the case of wildcats using purely the geometry and material properties of the chamber area and it still does a decent job. It's not cheap.

Though a chronograph alone is a lousy indications of pressure using a chronograph along with internal ballistic software like Quickloads or GRT can give you a rough idea of the pressures, better than trying to read primers (or anything else post firing) that is for sure.

twarr1 said:

Just keep in mind max peak pressure can be much higher than average max pressure.

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Yup. Seconded. That's where projectile, powder, primer and case choices come into play.

mcb said:

There is a commercial product that can do what the OP wants. It's called Pressure Trace (https://www.shootingsoftware.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=RSI&Category_Code=PT) .

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Great post, mcb! At first I was skeptical of mfgr claim ….sampling rates of 1 every 10 microseconds. But I see with strain gauges, that is currently being met by lots of mfgrs.

Ru4real said:

Great post, mcb! At first I was skeptical of mfgr claim ….sampling rates of 1 every 10 microseconds. But I see with strain gauges, that is currently being met by lots of mfgrs.

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Yeah 100,000 samples/sec is pretty easy to attain for Data Acquisition systems now a days. You can get DAQ that will do 500,000 samples/sec for $100 that plug into a USB port or get 100,000 samples/sec running on a Raspberry Pi. Digital Data Acquisition is getting pretty cheap and accessible.

Muzzle velocity is highly correlated with peak chamber pressure, with one important exception. I've run graphs like this for several different cartridges and the result is always the same. You can use muzzle velocity as a good measure of relative peak pressure. Note that the correlation is 99%. This particular data set is from an 8mm Yugo Mauser.



Here's the important exception: As you increase powder charge, it is not uncommon to reach a point where additional powder does not produce additional muzzle velocity. As long as you are operating below this plateau, the correlation holds extremely well. You should always operate below that plateau anyway.

If you are using the "book" specified powder and bullet weight, and getting MV at or below the "book" MV (compensating for barrel length), your pressure is also at or below "book" pressure. If you're getting more than "book" MV, you're over "book" pressure. Powder type, powder charge, bullet weight, seating depth, neck tension, etc. all drive pressure and pressure drives MV.

The Case Head Expansion myth is firmly entrenched. Here is a scatterplot of actual measured pressures vs. the corresponding CHE. Data are from one batch of brass, measured in a fixture to ensure that the same point on the case was measured before and after firing, using a Mitotoyu digital blade micrometer with verified resolution to half a ten thousandth of an inch (50 millionths).

Use CHE at your peril. I have a table of random numbers I will loan you that is almost as good. You can use that and avoid the inconvenience of measuring and firing.

Case head expansion is somewhat useful in a very narrow way, and it needs to be very carefully done. It really is only good on new brass of the same lot. The basic idea is to mark the case heads with a fine sharpie or something similar and then measure on that mark every time. If you or your tools cannot repeatedly measure to .0001", then don't bother. You'll only be kidding yourself.

If you've made it that far, then you can begin loading and shooting. Give each case an identifying mark, like a number. Note the unfired measurement from each case. Load a set with a starting load and place each round into the chamber with your measuring mark aligned with some feature, like 12:00 O'clock, and fire them. Remeasure. Look up expansion numbers for your type of cartridge. Low pressure loads will usually max out at .0003 to .0004 expansion while Magnums can go up to .0007". Most cartridges fall somewhere in between, and you really do need to know what to expect from yours.

All of the above also can be thrown out of whack with very soft or very hard brass. And even when you have accurate numbers, they are somewhat open to interpretation. If your .270 shows .0002" expansion and you don't have any other pressure signs and your velocity is roughly where it ought to be, you're almost certainly fine. What if it shows .0005" with no other issues? Safe? Borderline? What if it's .0003" but bolt lift is hard?

I personally tried all of it way back when and decided the method just wasn't concrete enough. At best, it would show me too much expansion and, in conjunction with other pressure signs, tell me that my load was too hot. By how much was anyone's guess - and guessing is all I was really doing, even compared to the guy who slaps a .001" dial caliper around random places on the case and declares himself to be fine.

For my money, their are two ways to not be guessing about pressure: A) buy a strain gauge set-up and learn how to use it, or B) stay with information from the manuals, working up from minimum and paying attention to the chronograph.

<edit> @denton posted while I was writing, and his graph doesn't surprise me. I would ignore case head expansion as a reliable sign of anything other than gross overpressure.

Spare Parts said:

Is there a practical way for the casual reloader to measure chamber pressure? CUP? PSI? Look at headstamps? primers? -seems like reading the entrails of pigeons. .

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Before this really goes down the really deep rabbit hole how about a simple.....

No, there is no way.

Rule3 said:

Before this really goes down the really deep rabbit hole how about a simple.....

No, there is no way.

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Well, I'll agree that a home reloader is not going to make good pressure measurements without some kind of strain gauge equipment. But I don't think that the situation is quite as dire as all that. Almost, but not quite.

QuickLoad does a decent job if you use real measurements of case volume, etc. instead of the defaults. It is certainly much better than CHE (which is setting the bar awfully low), and maybe even about as good as CUP (not a very high bar). So there is that.

And a muzzle velocity measurement can give a simple go/no go comparison, which is the most primitive of measurements. If you are loading below the point that MV plateaus with increasing powder, and if you are using the "book" powder, charge, and bullet weight, and if your MV (corrected for barrel length) is below "book" MV, your peak pressure is also likely under maximum. Pressure drives MV, and a whole bunch of things drive pressure.

I do have a couple of pressure measurement systems, and haven't used them for a long time. They were useful for working up loads in a modern 6.5x55 and a 7x57, and for resolving some discrepancies in possible 243 loads. But essentially they found no surprises.

A chronograph, on the other hand, is fairly cheap and incredibly useful. It is a good idea to work up your loads, but not for the reason most people think. You won't usually see "pressure signs" until about 70 KPSI, so you're probably not going to run into those with sane loads. What a progressive workup will tell you is whether or not you are operating below the plateau, and that is important information.

Spare Parts said:

Denton - thanks for Actual Science ® in previous posts. As a recovering (retired) Engineer, I groove on numbers.

Edit:


OK, I just re-read Denton's post. This matches what I know, plus a little more. Thanks again.

Previous posts mention OAL, and I agree with those. The only thing I could add is that we assume OAL is constant, since that would vary the pressure. PV = uRT.

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A real minor point of correction. Since we seem to be having more threads lately with people asking about load data for bullets other than what they’re actually loading, it might be good to remind folks the actual OAL isn’t what effects pressure, it’s the case volume after the bullet is seated and the powder column we’re actually talking about when it comes to pressure. Knowing the OAL of the bullet you’re using and the bullet you’re pulling data on is real important.

Not sure about casual but this is cheaper than doing burst disks in a test barrel, if you don’t have machine equipment..,

https://www.shootingsoftware.com/pressure.htm

Despite all the potential benefits of the Pressure Trace II measurement system, the cautions below are printed in their Operation and Instruction Manual. Proceed as seems wise.

"PressureTrace II will help evaluate small arms ammunition but should not be used to determine ammunition safety."

and

"Never exceed the maximum powder charge indicated by the powder and/or bullet manufacturer."

Yeah, lawyers speak. You will find similar information in your firearms manuals.

For example, if you follow the warnings in your Glock 21 manual, you wouldn’t be shooting reloads in the first place.



Of course such warnings are not limited to just the firearms industry. This label,



Is intended to be placed onto a device that itself creates both sparks and an open flame. The CPSC will even issue a safety alert if they don’t stick it on there.

https://www.cpsc.gov/Recalls/2007/s...ose-gas-lighters-sold-at-wal-mart-lack-safety