Do the shadow graphs show anything of interest?

To the best of my knowledge, nobody has done shadowgraphs of this phenomonom. It might be revealing, but, as far as I know, none exist.

There was a very hot issue a few decades ago, about guns blowing up with reduced loads of 4831. I've wondered if somehow that is connected to this. OTH, there are curve traces of reduced loads that distinctly show very sharp internal spikes in the pressure curve.

Good question... but I don't have anything to contribute.

The number of unanswered questions I have is expanding faster than my supply of answers!!

I wonder....



I've never been able to figure out why reduced loads of 4831 sometimes blow up rifles, or why the early pressure spikes are so "dang" fast. In the erratic loads, you see very definite fast, short spikes superimposed on the normal smooth pressure curve.



Hydrogen is one of the products of powder combustion. Is it possible that in a half-full case, there is enough oxygen left to create a little "bang" that would generate the fast spikes?? Doesn't seem like there would be enough...

Here is some nice pressure curves from my .338RUM.


This load is a Hornady 250grs with 86grs MRP, the first shot that has the big spike was with a clean barrel.


This is a load with a 225grs Norma FMJ and 91 grs MRP

I think it is cool that after 110 years give or take of using smokeless powder and jacketed bullets we still don't know what is going on in there!
On the other hand, it drives me a little bit to distraction.
JCN

The first thing to establish is if this is real data. It could be a transient caused by a shock in the wiring.
When a wire is moved there is a redistribution of the charge (rapid time base, "tribo-electric" effect)).
Early strain gage/chamber pressure set ups would sometimes show, for example, a 10,000 psi spike when
the firing pin fell.
It would only take a second to tape the wires down and repeat the measurement.
Secondary pressure excursions were described as wave reflections in a closed barrel, this one is hard
to visulize. It is hard to believe that an open ended gas explosion would create a 80,000 psi pressure at
the breech.

irv's idea isn't a bad one, by any means... Here's the list of possible sources that I've been able to come up with, all of which you guys have also covered:

1. Gas re-ignition. Further complicated by indications that the little + on the trace that indicates bullet exit may be late.

2. Electrical phenomonon. Powder gasses are really a plasma, which can show electrical effects. Strain gage signals are millivoltish, which means that they can be interfered with easily. Leads are well shielded, and the circuit board is carefully done to protect the circuitry, but who knows what else might be going on.

3. Barrel vibration. Could be longitudinal, transverse, or torsional. I used to think this was the explanation, but it's looking less likely.

4. Some kind of internal oscillation/pulse in the gas. Includes the theory that somehow the "bullet gets ahead of the gas", or it might be some kind of "pop" that happens when the bullet exits the barrel.

Robban's picture shows a big pulse with a clean barrel. That's an interesting find. I haven't seen that before.

Truly, there is a lot we don't know about what's happening. About the only thing I'm convinced of is that the pulse is really there, and trying to tell us something.

Another wild thought I had and discarded as too far out was the electrical interference...but maybe I was too quick to dismiss it. We have a current in a wire, and a device to move it rapidly in the form of a shock wave from the muzzle blast. Moving a wire with a current flowing causes a shift in that current...hmmmm I am really curious to see if you can use an optical sensor to really determine the bullet exit point on the pressure graph. It will tell a lot, IMHO. The wire interference can also be checked by positioning a second trace on a rifle beside the one being fired. If a pulse appears on the second (unfired) unit, something is causing a crossover and I think electrical effects would be the primary suspect.

I still lean towards the flexing of the metal during the firing/recoil process causing it. We know barrel whip is real, and that much vibration has to cause some flex at the shank of the barrel. I would be interested in comparing the accuracy of loads with high secondary peaks with those that don't show much in the same rifle.

Any idea when you might be able to get out and test these theories?

I tested those loads before i bought my PT and i concidered them as safe, The only thing that i saw was that the primer was a bit flat but that doesnt always mean that there is a overpressure.

I have got the data from Norma and they are about a grain less than the max loads.

What is the barrel length on the rifle used to produce the graphs? Does it have a muzzle break?

HogWild

Denton,
Another clue could be ; Why does this "pulse" occur with pressure trace and not other strain gage or
piezo-electric measurements?. The fact that this is the first time in 110 years that this has been
observed is interesting if no alarming.
Take Care!

irv...

The other strain gage system I have is the Fabrique, which just gives you a peak number. Their instruction manual mentions "barrel vibrations after bullet exit", and that product simply suppresses them. So two of two different strain gage systems have the effect.

There is mention in the literature about gas re-ignition, so I suspect that some of the early piezo people saw the spikes, and attributed them to that... don't know, though.

Two experiments:

1. Is it some sort of electrical artifact? Test: Make two small pieces of brass, about half the size of a strain gage, and connect them with a 350 ohm resistor. Attach to a barrel. The PressureTrace will think this is a strain gage, and balance the bridge. Fire some shots. If there is some kind of electrical pulse that accompanies a gun discharge, this will detect it. Alternatively (more work) hook the barrel up to the input of an oscilloscope, and touch one off.

2. Is it gas re-ignition? Test: Use the old muzzle flash suppression trick from artillery applications. If the pulse goes away when muzzle flash goes away, then it is probably gas re-ignition.

Well - what the strain gauges actually measure is the resistance change in the sensor strip due to the radial expansion of the barrel caused by the pressure inside the barrel.
1) I don't see how any kind of longitudinal vibration bending of the barrel (typical barrel "whip" seen in high-speed photography) is going to show up as this vibration doesn't cause the barrel to expand radially. Thats why the sensors have to be put on across the barrel length, so this doesn't happen.
2) The Radial expansion of the barrel loads the barrel like a spring, and when the barrel contracts it hits a minimum, then springs back radially and causes an expansion that is picked up as the second pressure spike. Problem here is that the minimum should show up as a negative pressure if it is going to re-expand past the normal no-pressure rest state of the barrel. I don't see a negative pressure in any of the traces. Also the second spike is sometimes bigger than the first, not possible if this second spike is caused by a spring effect - the second bounce is always smaller than the first.
3) Re-ignition of the hot gases when they hit the atmosphere. How do you get significant pressure (80,000 psi) back at the chamber when the re-ignition happens past the end of the barrel in an unconfined reaction? Air has very little inertia - way too little to contain 80,000 psi.
4) Pressure wave inside the hot confined gases gets reflected off the bottom of the bullet and causes the pressure spike back at the chamber. What causes the pressure wave in the first place? First pressure impulse that happens is the primer going off. Enough energy to ignite the powder, and usually will move the bullet out of the case if there is enough free-bore, but either way the bullet will stop? when it hits the rifling cause there just isn't enough energy yet to engrave the rifling into the bullet. This first impulse isn't picked up by pressure sensors because it is below the sensor threshold (probably in the 2-300 psi range??). Second impulse is when the hot gases from the burning powder first hit the bullet base and pressure starts to go up. Is this an impulse, or is it a smooth rise? The pressure traces show a smooth rise, so maybe there isn't anything to cause an impulse wave? I repeat that a very long barrel (at least twice the length of a normal barrel) would prove this one way or the other. If a third spike shows up in a very long barrel, then it is a pressure wave reflecting one more time. If it still shows just two peaks in a very long barrel then it isn't a reflection.
5) Electrical or sensor malfunction caused by the stress of a shot going off. The impedance of the sensor, it's wiring, the currents and voltages used are small, but so is the signal from the sensor. This is a possibility, but we need to find out what the impedance of the sensor is, the voltage and current actually present when the shot goes off to see if this is the cause. I doubt it, because how could anyone trust the sensor signal for the first part of the trace, if the big second spike were caused by these electronic effects?


I am curious about one thing in the traces shown - I thought that when the bullet exited, there would be a sudden drop in pressure in the barrel that would show up in the trace. Instead the pressure drop looks to be fairly smooth as the bullet exits. This doesn't make sense to me. If the gases are still pushing the bullet (and they must be because a barrel a couple inches longer produces a little higher velocity), when the bullet no longer blocks the bore the gases should move outward without restriction and the pressure should drop faster than when the bullet was restricting the gases. The graph should have a "kink" in it at the point the bullet exits the barrel. I can't explain this either.

I don't get a chance very often to look follow these threads or I might have been able to jump in sooner. Let's see if I can answer a couple of the questions.

irv said:
[Why does this "pulse" occur with pressure trace and not other strain gage or
piezo-electric measurements?. The fact that this is the first time in 110 years that this has been
observed is interesting if no alarming.]

If you look carefully at some traces made by the Oehler system, you'll find that they do, in fact, show an attenuated secondary spike. The problem is that their system is an older, slower, design and can't catch the entire event. I suspect that their system does some filtering and smoothing that will tend to minimize the peak even further. Steve Faber filtered out any peaks he didn't believe belonged in his system.

CMcDermott and CDH raise some issues. Instead of trying to answer them individually I'll take a different route.

When I first saw the secondary pressure spikes I obviously questioned them. I went round and round in discussions with Denton (and some others) to explain them. Here's how it finally ended up.
1. It's not barrel vibration. We eliminated that experimentally.

2. You can make the spike worse, or make it go away by changing the bullet weight or the powder (speed) or the barrel length. That eliminates a problem with the electronics or the software. It also pretty well eliminates the idea of the electrical pulse being generated by the firing cartridge.

3. Charlie Sisk, inadvertently, performed a test for us.
He ignored the secondary spike measurements he was seeing from PressureTrace while working on a 338 Win Mag load. He upped the powder just a bit more and blew the end off the barrel. After cleaning up the barrel and recrowning it at the new, shorter, length, he no longer gets a secondary pressure spike with that barrel.

Conclusion:
While the reignition effect may seem unlikely, it is happening.
When I see a 70K PSI secondary peak, measured at the chamber, it makes me very afraid. When I'm working up a load and I see evidence of a secondary pressure spike, that bullet and powder combination is immediately dropped from further consideration.

For instance. AA2230 and a 70 grain bullet is a nice combination in a 6PPC. When I tried the AA2230 with 40 and 45 grain bullets in a 223, I got big secondary spikes. AA2015 (XMR2015) with those same bullets in the 223 shows no secondary pressure spikes.

CDH. Buy a PressureTrace and run it for awhile. Take it apart and have a good look at the electronics. That's a 4 layer circuit board using high quality parts with some extremely careful circuit layout. This was not designed as a consumer product. I've been at this for nearly 30 years now. Much of that time working on systems where normal "industrial" measurement systems werent good enough. I worked on that circuit full time for more than a year before it became a product.

By the way. It is a differential input from the strain gage, but there is no hardware or software rectification of the signal. If the signal goes below zero, it is limited to displaying as a zero.

More information is in order.

Reignition, Deflagration, Detonation:
Per common usage, deflagration is what normally happens in a firearm. I don't know that we want to call whatever it is that is causing the secondary spike detonation. I've started using reignition as a catchall term. It could be used to cover the reigniting hydrogen theory or the theory that I personally prefer. That theory is best described as the catch up theory. It goes like this: When there is a mismatch between the powder burning rate and the bullet weight (or bullet bearing surface), the bullet accelerates fast enough down the bore that the pressure reduces to the point where powder burn is no longer happening as fast as it could. Pressure drops and the bullet slows down. The expanding front of the burning powder gasses then catches up with the bullet. Pressure increases and the powder starts the burn energetically again.

Is that theory perfect? Probably not. However. Using that theory I can use a faster burn rate powder, or a heavier bullet, or even a bullet with a larger bearing surface and make the secondary spike disappear. It seems to me that if I can control the parameters used by the theory and control the output condition based on what the theory says, I can't be too far from the truth.

Bullet exit point.
Denton states it well when he says that the bullet exit point marked on the graph is an estimate. The formula currently in use to calculate that exit point comes from Julian Hatcher in a book written in the late 1930s.

Based on work by a customer (Chris Long of Omega Tau Systems) who took the considerable time to work it out with MatLab, we are testing an implementation of marking the bullet exit point that should be much more accurate. It uses bullet friction calculations based on the bearing surface, the weight of the bullet and the pressure changes down the barrel. It also uses the muzzle velocity entered by the user after the shot is fired.

To test the new bullet exit algorithms I modified two PressureTrace modules. One starts running when it is given an external trigger signal. The second one outputs a signal when pressure reaches the trigger point. (On the newest, USB units, it is possible to run at a trigger level of zero. Any increase if pressure that lasts for at least 500 microseconds, causes the unit to trigger.)

I then mounted a strain gage at the muzzle of my 30-06 and connected it to the externally triggered PressureTrace.

When I fired the rifle the first PressureTrace collected a normal pressure curve and had output a trigger signal at time zero. The second PressureTrace showed a flat line until the bullet went under the strain gage at the muzzle.

What we found was that the old bullet exit algorithm was marking the bullet exit point about 110 microseconds too late. The experimental data agreed exactly with the bullet exit point marked by the new bullet exit algorithm. FOR THAT RIFLE.

When the algorithm has been tested further and proved correct, we'll implement it in a new release of the PressureTrace software.


Barrel Vibration:
This was one of the very first theories we looked at. We eliminated it by shooting the same load with an undamped barrel and then by putting a heavy sand bag on the barrel near the muzzle. When the traces were compared, they looked exactly the same.

If you had a free running system, as I've done a few times, you would find that you can start the system then whack the end of the barrel with a chunk of 2x4 and the strain gage will not see any expansion of the barrel at the chamber.

As the distributor of this device I try to stay out of these discussions...especially where Hot Core is concerned...and let customers followup. Somehow the forest is getting lost in the trees so I will chime in at the top of everything that followed.

1) SAAMI protocols are established for ammo manufacturers not reloaders (hence the organizations name). Only if your rifle was chambered with the same reamer will test ammo produce similar pressures in your rifle.

Experienced reloaders seat off the lands and your chamber will not be cut to SAAMI case specs. Pressures in commercial rifles are typically lower then in a SAAMI spec test barrel. Remember when load manuals were based on test barrel data and never came close to "real world" performance?

2) Strain gage technology is well established and has largely replaced older instrumentation methods in industry. Any debate about it's ability to accurately forecast pressures is easily dismissed by applying strain gages to a SAAMI test barrel. Guess what? Strain gages will consistently produce lower errors and greater shot to shot accuracy. This is good news for shooters who prefer not to drill holes in their barrels :-)

3) Peak Pressure is a secondary aspect of PressureTrace. This device should not be confused with other peak pressure meters that have been sold in the past. It captures real pressure data with sufficient resolution to overlay traces directly on top of each other. You can see the affects of barrel fouling, neck tension, seating depth, etc. on the pressure curve as well as the performance of various components. That is why it exists. It was developed for shooters to measure pressures and refine loads, not as a means of testing extreme lot variations in commercial ammo.

4) Secondary pressure spikes are a well known phenomenon, at least by those who formulate powders. It may be less understood by those who redistribute powders and is certainly poorly understood by the typical handloader. Prior to the introduction of PressureTrace, handloaders could not see it for themselves. In defense of those who assemble load manuals they have no way of knowing what the barrel length might be, if you shoot moly, and what bore friction a particular bullet will produce. Yes, just the brand (shape) of a bullet can cause secondary spikes that do not appear with other bullets.

5) Denton Bramwell's great series of articles on pressures published in Varmint Hunter Magazine, movies showing how gages are installed and the PressureTrace software are all available at the RSI web site. I would like to add much of Bramwell's work was published before PressureTrace was introduced and before we became aware of his activity.

Bramwell Articles In Varmint Hunter

RSI does not recommend reusing gages removed with acetone because they can be removed in tact only about 50% of the time. Even if a gage is successfully removed it may get "kinked" so it's resistance (gage factor) value is altered.

The encapsulated gages are easiest to put on and extremely small if excess carrier tape is trimmed back to the gage. When placed on top of a barrel under a scope they do not interfere with normal field use.

A better alternative to removing a gage if they are deemed "unsightly" is to use the bare pre-soldered assemblies. These are offered for custom installations so they can be located on the side of the barrel (or underneath) where the gage might be hidden and only the plug shows.

So you are telling me that with all the research you guys have done the best guess is that the bullet is outrunning the force driving it down the barrel!?!? Sort of disagrees with Newtonian Physics on the surface...maybe the powder is burning through the deterrent coatings used on slow powders and igniting the interior layers REALLY fast, but that seems too fast of a risetime on the spike...


All the sandbags on the barrel and whacks with a 2x4 check is longitudional stress and springback, my original theory was/is radial. I know, I am hard to convince...one of these days I'll have a couple hundred to blow, but right now I have too many expensive gadgets. My wife would make me sell one to get a strain gauge, especially if I told her it was to help with a discussion on an internet forum!

Fun stuff. Too bad the other side doesn't see this side of the picture!

Unfortunately there were a number of statements in the article that were incorrect or confusing. Piezo systems often attempt to calibrate out the pressure held by the brass, which can be significant. This is useful information for systems that measure pressure through a hole in the chamber and brass. 5,000 PSI or more can be held by the brass so it is of great interest to commercial manufacturers of low pressure ammo.

The characteristics of brass and steel are well known. Strain gage technology can measure the expansion of thick or thin wall vessels (brass and barrel). Unlike other systems, PressureTrace calculates both the pressure held by the brass and by the barrel. This is why there is a required parameter for the thickness of the brass. Unlike another system you do not enter a general "catch all" number for brass strength.

The fact there are now three systems in use for measuring chamber pressure no doubt is causing some confusion for writers. Ammo manufacturers will likely continue using their standardized system. The two strain gage systems now available to shooters are quite different in design and do not use the same methods to calculate pressures. All three can and do produce similar results when properly used.

Laws of physics actually hold nicely with this theory. Here's why. The area under the curve is the energy delivered to the base of the bullet. There is greater initial energy to the peak pressure point, then it drops significantly. This initial pressure "bump" establishes the rate of acceleration through most of the barrel. As the bullet travels down the bore, friction will slow the rate of acceleration...but the bullet is still accelerating. As the rate of acceleration decreases gass can catch up and "Whack" the bullet on the base. This is still the best expanation anyone has devised...especially given how easy it is to create or eliminate secondary spikes when applying the theory.

There are indeed longitudinal wave forms that travel down the barrel. Chris Long and others have discussed this in detail here and on other forums. There is also spiral whipping of the barrel. These all relate to "harmonic nodes" but are not picked up by a gage wrapped "around" the barrel as you do for sensing chamber espansion.

Denton and others are still looking for other possible explanations but if hanging 35 pounds on the end of a barrel does not alter secondary spikes it is probably not related to barrel pulses or whip. Secondary spikes can be introduced in any caliber by shooting lighter then normal bullets or a slow powder. They can be eliminated by shooting a heavier bullet or going to a faster powder. Even if the theory is faulty, it is still seems to be the best guess. Stay tuned.

Fortunately these secondary spikes are usually not severe enough to cause problems if people are using a reputable load manual. normal barrel lengths and standard loads. They tend to be a bigger problem for those pushing the envelope with long barrels, wildcats, super velocities, moly, and light bullets.

People regularly ask if they should be concerned with a minor secondary bump. It is my belief if the secondary pressures are under 35,000 PSI the chance of blowing off the end of a barrel might be slim....but it is not a good load and will often be one of those, "drives tacks today, shoots like crap tomorrow" loads.

CDH is asking all the right questions. I will add a bit more about what you are seeing. The event is occuring in the final few inches of the barrel. The gage is attached over the chamber. The gas column in the bore may act as a long gas shock absorber and the thinner parts of the barrel will expand to absorb some of the pressure before expands the chamber steel under the gage.

In small 22 varmint calibers a secondary spike of only 65,000 PSI will often produce sticky brass. The pressures are actually higher, but of also shorter duration then indicated on a trace captured at the chamber. This also explains why some big bores show secondary pressures off the scale without sticky brass.

If you place a gage before the area where the event occurs, indeed you get a verticle blip when the bullet passes under the gage and higher pressure of shorter duration.

Good thought, but we do run two units at the same time to verify barrel timing calculations and for other tests. If you saw one of the units you would see it is extremely well shielded inside and out. The unit passed FCC testing on the first try... which we were told by the lab is unusual. It is not electrical noise.

We believe the same thing is seen by other systems running at a similar sample rate. Unfortunately other older systems have a much lower sample rate and often run data through a smoothing algorithm so it does not look as severe. Because some systems are too slow to keep up or remove that part of the curve does not mean it is not there.

Technology marches on to help shooters :-)

Burt, Jim... did I mention that there are some very competent and inquiring minds on this board?

I thought that the Handloader article went astray on a couple of points. I have a letter in to John Barsness that might be the start of a worthwhile conversation. Let's see what he has to say.

We've talked about these issues offline. I have a lot of trouble imagining the bullet getting ahead of the gas. Of course quantum mechanics does the same thing for me. That's why you have to trust the data, not intuition.

Since we have to rule out some causes, smacking the barrel with a 2x4 is a good test. I would not expect it to register anything, since the strain gage is made to reject any transverse strain. I would be amazed if directly smacking the muzzle across, or "head on" produced anything. The strain gage should reject those modes even though the barrel will vibrate.

Having thought about the data, my opinion is that we can rule out all barrel vibrations except torsional, and I'm leaning toward ruling that out, too.

rsi and Burt,

I sure appreciate you guys taking the time to post and give the board some "inside" info. I am very impressed with the Pressure Trace product and look forward to "playing" with my own.

The theory you've presented is interesting. If true (and it's difficult to argue with parameters behaving as the theory predicts), I would guess the bullet seperates from the gasses the moment it obturates and "squirts" through the lands. Then the gasses must accelerate from essentially a stationary speed to a speed greater than the bullet if the pressure indeed spikes before the bullet exits the barrel.

I've got to say that the theory fits. More likely for the pressure to "catch up" and build in a long barrel, and if the barrel is shortened past the position of the bullet when the pressure begins to spike, it is obvious why the spike is eliminated.

Slow powders are more likely to cause the secondary spikes? Is the theory that since it burns slower and there is typically a higher charge weight, there is more likely to be unburned powder to initiate or contribute to a secondary spike? Do you think the unburned powder (if any) is traveling down the barrel and reigniting near the muzzle? If so, does unburned powder get pushed out the muzzle if the barrel is shortend to the point the secondary spike disappears?(assuming the same load)

Is there a certain length barrel where one typically runs into problems? Have you seen problems using 24" barrels?

Thanks again for the input. It's obvious you have a well thought out product. I've not used the Pressure Trace (yet) but have used the Millenium chronograph and it's the best I've used.

HogWild

Burt
I dont usually post over here, but I have to jump in...
That test with the 338 DID happen...not once, I was crazy enough to do it twice....got the same results both times...I aint gonna try for three.
I have seen a lot of weird things since I got this system. Secondary spikes are real in my mind, I can make them higher, lower, shorter or longer in duration as I want. I have beat the end of a test barrel with a 1 1\2 pound sledge and it doesnt show a secondary spike ! I cant explain why they happen and I really dont care. When I see this, its times for a different powder, bullet, or primer.
A few odd things I have learned...
1. pressure and velocity DO NOT go hand in hand...high pressure does not always mean high velocity and low velocity doesnt mean low pressure...
2. In my 8x57, at 60,000 psi, that rifle will shoot a 220 grain Hornady at 2440 to 2465.....with Reloder 7, Reloder 15, IMR 3031, 4895, 4064, H4895, Varget, 322, BLC-2, Benchmark, Ramshot TAc and Exterminator......makes me wonder why I need all these different powders...
3. Flattened primers and sticking bolts are NOT a sign of high pressure...let me explain.....you have what I call axial pressure (bolt thrust) and radial pressure....I have developed several loads that will stick a bolt at about 42000 psi.....go up to about 50000 with the same components and it goes away....obviously you dont want a load that sticks the bolt, but that doesnt mean high pressure in the barrel...I dont know why this happens and dont care, I just dont use those loads.
4. Primers make a heck of a lot of difference...sometimes....sometimes none at all....sometimes magnum primers will produce less pressure and velocity, sometimes higher pressure and same velocity, sometimes higher on both.....
5. The most important thing I have learned after reloading since I was about 14 years old is, if you dont have some method of measuring pressure, you really are just guessing
I'm sure this post will have some folks thinking, this guy is full of sh!#. ..and I may be.. ...I've had several folks tell me that....This past year at the Houston Safari Show, a man came by and told me I didnt know what I was talking about and said "I'll have Craig Boddington come over here and straighten your a$$ out". I said "ok, Mr Boddington is much more experienced than me, hopefullly he can teach me a thing or two." I never got to meet him until about 3 months later... ....He's an excellent speaker.....heard him at the Houston monthly meeting...I'd like to go buffalo hunting with him sometime...
I'll leave it like this....I have talked to most of the powder companies and there are a lot of things that just are not known when it comes to what goes on inside a barrel.... if someone has some information supported by data and not ideas, I want to hear it...I bought this equipment to learn...I dont this to come across as a smarta$$, thats not what I'm trying to do here...I have learned enough now to know that before I had this I didnt know sh!# !
Charlie Sisk

The "pulse" could very well be a bending moment in the barrel. The action has only moved bach about 0.1 in by the time the bullet leaves the muzzle. The moment however has been applied from the time the bullet started to move.
The moment is caused by the fact that the centerline of the barrel is above the butt (that's why the gun rises in recoil). It is accentuated by the fact that the recoil lug
is below thr receiver. What happens is; The receiver moves back, and up. Inertia hold the barrel in place causing a
bending moment at the barrel receiver junction. Since the
receiver not the barrel is moving no amount of sand baging ect. will effect the measurment.
What is required is to use more than one gage in a pattern that will measure hoop stress, and cancel moment.
Good Luck!

Interesting that you mentioned Norma. Some 20 years ago I first heard of the "catch up" explanation (as posted here by Burt and myself) from a Norma ballistic engineer named Nils that used to spend time in the states. Perhaps others on this list remember his last name which escapes me. I understand Nils died quite young of a heart ailment.

I have been a proponent of this theory ever since and give full credit to Norma as providing my first exposure to the theory. Nils explained their instrumentation could not collect consistent data (days of slooooowwww processors) but they believed the secondary spikes where real, often quite severe, but of a duration to short to measure with instruments of the day. They did not have proof positive and could only postulate theories at that time. He used this as a possible explanation for why people had catastrophic failures shooting light bullets in a 30-06 with the original Ball C.

Nils explained that a highly compressed gas behaves more like a fluid and often compared it to bad plumbing in a house, where when the water was turned off quickly, pipes would rattle a long way away from the faucet. Perhaps those more familiar with fluid dynamics could add more, but Nils' analogy stuck in my mind.

If someone at Norma knew Nils, they may be able to use his time of employment to locate early research in this area. Anyone on the list remember Nils?

Hey guys,

There is a fellow named Harold Vaughn who wrote the book Rifle Accuracy Facts . He also writes for Precision Shooting and The Accurate Rifle . He was the chief of the Aeroballistics Research Division at Sandia Laboratories on Kirkland AFB in Albuquerque, NM for about thirty five years. These questions are right up his alley, and in the middle of his field of expertise. He may even have written articles about this phenomenon. I'm actually working this week, but if somebody would forward this thread to him for evaluation and commentary I think we would learn an awful lot.

JCN

Harold's book is very well founded on fact, and a pleasant read. (Couple of errors, but an excellent book.) He's just the kind of guy you'd want in this kind of discussion--wouldn't be surprised if he would have some data or knowledge that would immediately resolve the issue.

Unfortunately, when I tried to contact him, c/o the magazine, I found that he has recently departed the planet. Really makes me wish I had found him a year ago.

Sorry to hear that. He pulled himself up in life by his bootstraps. His contributions to our victory in the Cold War were profound, and must go unsung.
JCN

Denton: I am curious as to what errors you found in Vaughn's book?. He was the first person to explain wind drift of bullets in a logical manner. I called him about a year ago he sounded weak then. Good man!
Take Care!

Don't get me wrong... I think it's one of the best books on gun accuracy I've ever seen. It's a really good piece of work. I really wish I had gotten to know Harold. He just sounds like a really neat guy. Anybody who would haul a Tek 555 tube type oscilloscope around in his pickup, with a gas generator to run it, is what I call a hard-core experimenter.

There is a mistake on page 145, where he observes a small improvement in group size. In order to do the RSS thing that he did, you must first show that the change is not easily attributable to random variation. The data would not pass such a test. The correct interpretation of the data is that the change was made, and there was no detectable difference as a result.

It's a real error, but not something that keeps me up at night.