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Sisk Rifle, c Nick Leghorn

When a gun goes off, the expanding gasses created by the burning gunpowder pressurize the chamber and force the bullet down the barrel. There’s all sorts of interesting science behind what the maximum pressure of the ammunition can be before the chamber ruptures, and what the proper pressure is for the best and most consistent grouping. But that all assumes a “normal” pressure wave. What happens if your ammunition isn’t actually conforming to that normal curve? And is a secondary spike even possible? . . .

Let me start by talking about measuring those spikes in the first place.

The standard method for measuring the pressure in a firearm’s chamber is by direct observation. A hole is drilled into the chamber, and a small sensor is placed in the hole. Historically the sensor was made of either copper or lead, but more common today is a piezoelectric sensor that translates pressure into electrical current.

The problem with direct observation is that it’s expensive and you have to sacrifice a barrel to the process. However, Charlie Sisk at Sisk Rifles has a pretty smart way of doing pressure testing on the cheap. Instead of directly measuring the pressure in the barrel, he simply measures the expansion of the metal surrounding the chamber (since the metal deforms slightly as the gun is pressurized) using some very accurate sensors and computers. Through years of testing, he has perfected the formula for determining chamber pressure based on metal expansion.

Normal pressure, c Charlie Sisk

This is a “normal” pressure curve. The pressure in the chamber increases as the powder burns, then decreases as the bullet moves down the barrel and increases the available volume. Eventually the bullet leaves the barrel and the pressure drops off.

However, Charlie started to notice that sometimes when he was working up a load he’d get a secondary spike. One that didn’t make any sense.

Secondary spike, c Charlie Sisk

You expect the first spike from the initial gunpowder burn, but where the hell did that second pressure spike come from? It didn’t make sense.

Naturally, Charlie kept testing. As a result, he can create a secondary pressure spike whenever he wants just by varying the load parameters. The ability to repeat the results on demand indicate that this isn’t some kind of instrumentation fluke. There really is a secondary spike going on in some loads. And not just in handloads — Charlie has seen this same pattern in some commercial ammunition, too.

“So what?” I hear you say. “That’s interesting, but what does that have to do with me?” Well, turns out that secondary spikes might have some safety implications.

Installing a barrel, c Nick Leghorn

Charlie was playing around with a new, full-length barrel (26 inches, I believe) in his gunsmithing shop and creating some secondary spikes in order to figure out what was going on. After the seventh round, the shot sounded… different. When he looked, it turns out that the front five inches or so of his barrel had fallen off.

He repeated the same test with three more barrels, and all three were cleanly chopped off at 21.6 inches exactly. Secondary pressure spikes caused spontaneous barrel shortening. Or, put another way, they blasted off the front of his barrel.

The cause of these spikes is still unknown. But that doesn’t mean there isn’t a theory. The best of those is that the water vapor in the atmosphere of the barrel forms a “vapor cone” around the bullet, much like it does around aircraft breaking the sound barrier. That vapor cone, as the theory postulates, creates increased drag and therefore a pressure spike. Again, it’s still just a theory.

Charlie is continuing to do his testing, and others have picked up on the issue, too. There’s a paper on the subject currently being written by a couple of other internal ballistics boffins, so we should have some more hard evidence of what’s going on shortly. Until then, its just another fascinating mystery.

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  1. I noticed that even in the first graph, there seems to be a small increase in measured pressure after the bullet leaves the barrel. Since he’s actually measuring metal deformation, is that due to the reflected acoustic wave from the end of the barrel? And could that have something to do with the larger spikes observed at the same time?

    I’m no gunsmith, just trying to reason from basic physics.

    • Hmm – sounds like a standing wave – and it could be “tuned” based on the length of the barrel.

      • +10000. That’s exactly what I was thinking. Not enough of engineer though to say its true

    • You don’t need to be a gunsmith, this is fluid dynamics. In fact, this is one of those common examples of lack of general basic scientific knowledge. It happens all the time.

      Had the gunsmith any knowledge about movement of fluids (air is a fluid) he would have immediately run for a computational fluid dynamics program and started modeling this issue. Instead, apparently the screaming-like-Roseanne-on-a-bender clue of highly repeatable distance doesn’t ring any bells, and he gives up on science and goes for voodoo.

      He has the pressure and data acquisition tools right there, and yet he doesn’t do the most basic thing: start measuring pressure in other places on the barrel and plug the results into a CFD program.

      I remember long ago when the guys in F1 had figured out fences on the end of their wings to keep the air from spilling off generating more downforce. It took the drag guys almost a decade to figure that one out. They aren’t complete idiots, but they got overspecialized.

      “However, Charlie Sisk at Sisk Rifles has a pretty smart way of doing pressure testing on the cheap. Instead of directly measuring the pressure in the barrel, he simply measures the expansion of the metal surrounding the chamber (since the metal deforms slightly as the gun is pressurized) using some very accurate sensors and computers.”

      So does everybody else. Indirect measurement using quartz sensors has been around since the 60s. Strain gauge measurement has been incredibly cheap for decades and is also rather common among serious gunsmiths and handloaders.

      • His method for measuring barrel expansion doesn’t
        seem like it be very effective either. The difference
        in metallurgy alone would necessitate the creation
        of base numbers for each specific barrel. Different
        methods of construction would also pose a
        problem. Machine, cold hammered, fluted, lined,
        all would appear to produce a different pressure models simply by altering the measurable barrel
        expansion. It’s neat that he did this experiment,
        but as you say Sisk could have saved himself
        alot of time and trouble by lining the barrel with sensors and running a CFD.

      • I think characterizing a failure to run a computational fluid dynamics simulation as a “lack of general basic scientific knowledge” may be overstating the case a teensy bit.

        • As I noted in a different response that disappeared into the ether, you don’t have to be smart enough to do the math yourself. You’re right, that’s advanced stuff that few really have a handle on. But…

          You just need to know that it’s what needs be done (as well as the metallurgical forensics) and then have somebody run it. The heavy lifting of the math is in the program.

          This isn’t 1990. FEA and CFD programs are readily available, cheap or free, downloadable, and run on your desktop. If you don’t know how to plug the numbers in, hire a CAD whiz, a computer geek, or a grad student for a coupla c-notes. Modeling a bullet in a barrel is about as simple and straightforward as it gets.

      • ” In fact, this is one of those common examples of lack of general basic scientific knowledge.”
        – – –

        In fact, this is one of those common examples of losing an audience to a possibly valid and interesting comment by using the common but rarely-appropriate “you’re all idiots” opening.

        • “You are all idiots” was neither the intent, nor the wording by any stretch. But, if you chose to read it that way, you’re free to do so. The US ranks where exactly in math and science education on the international stage? I guess one should just ignore all the glaring examples of that continuing decline, eh?

          This ‘gunsmith’ was attempting to use the snake oil of “science-y” research as a marketing tool. When confronted with an actual failure, even pretending to play science went right out the window and he went for the voodoo. Because he didn’t understand basic science.

  2. Awesome post – I’m even double glad that I read it since I initially thought it was about a secondary spike in ammo PRICES! *smacks forehead*

    So glad to be an idiot.

  3. Maybe the second spike is caused by the bullet hitting the closed door at the end of the barrel in the top picture, which slows it down and causes all the gas behind it to slam into its backside?

    Maybe he can figure out how to induce the second pressure spike at somewhere between 10-14″, and then YAY HOMEMADE SBR!

    OK, that’s all I got. Neat article; that’s why I read this blog: the random neat stuff. Thanks, Nick.

    • In that you’re not alone. How is the stock market at its highest ever, and my 401k is the reverse?

      • Going back to last November and also in recent weeks, the Obama administration is looking to push getting rid of 401ks so Obama will eventually relieve you of your burden.

      • Because the funds in your 401k are required to invest their capital in accordance with whatever the prospectus says. Sometimes this puts the fund manager in the perverse position of buying more of a loser to maintain the position described by the prospectus. What do you do, when you run a Small Cap fund in an economy that is eating up small businesses? (Granted, due to extremely bad policymaking, but you get the idea). I review my 401k periodically, and if a fund is losing money it gets the axe. Period.

        I’m not series certified, and the above does not constitute investment advice.

  4. Mr. Leghorn,

    Unless Mr. Sisk is developing a modest velocity load for a shotgun or muzzle loader, I would expect that the bullet breaks the sound barrier in the first few inches of the barrel … well before the 21.6 inch mark. (For example 5.56mm NATO rounds have a muzzle velocity on the order of 3000 feet per second — well in excess of the speed of sound at 1116 feet per second — coming out of a 16 inch barrel. Thus those bullets break the sound barrier well before 21.6 inches.)

    An alternate explanation just popped into my head. Any time something happens so repeatably and accurately (at 21.6 inches) I think of resonances and constructive interference nodes. For the non-techies reading this, resonances refer to the natural rhythm of waves — such as the sound waves on a guitar string or waves in water. Anyone who has ever watched waves around a rocky shoreline or jetty has seen the waves sometimes add to make a much larger than normal wave — that is constructive interference.

    I wonder if something in the detonation and burn process creates a pressure wave inside the barrel which reflects back and forth (between the breech and bullet?) — and the reflections peak 21.6 inches away from the breech end? In other words is there a “node” at 21.6 inches where the reflected waves add and cause unexpectedly high pressures?

    I am not a mechanical engineer and I haven’t taken the time to look at the velocities and distances, but it certainly seems plausible at first glance.

    • When a repeatable, precise distance for the shear was mentioned, my first thought was a standing wave too.

    • Exactly. Fluids, compressible or not, readily acquire standing waves under certain conditions.

      That article should remind everyone how far our nation’s basic science education has fallen.

      • Science education at the level of an MIT undergraduate senior engineering major will never be achieved by a national system. And that is what is required to understand the mathematics behind traveling waves and solitons, and their eventual breaks or dissipations. In the example above a traveling wave or a soliton could be making a contribution to the pressure at peak one, but continue and break at peak two, barrel point X due to dimensional factors, resonance change. The wave is traveling, not standing, because the bullet is moving (and fast). Differential equations modelling interaction between barrel resonances varying as taper changes interacting with the basic pressure wave evolution would be incredibly complicated. Gilbert Strang, IAM, covers just the basics of the waves without barrel taper issues (without taper there would be no 21.6 chop off…). The level of mathematical preparation required to model such waves (or to work the problems in IAM) is rare even at MIT.

        • I don’t disagree with you at all that actually hand-cranking all that math out is very challenging and far beyond the capabilities of most folks. It’s certainly beyond mine.

          Which… is why we have cheap/free, widely available CFD programs. I don’t have to do that math, I just need to input the parameters. If one doesn’t know how, throw a few c-notes at a geek or a grad student. We’re not modeling complex curves in intake runners and interactions with cross-waves in a plenum at 8K rpms, this is a projectile down a tube. Easily tested velocities and pressures from start to finish.

          My point was that knowing a bullet down a barrel is fluid dynamics is basic. Knowing to throw more sensors on and acquire more data is also HS level scientific thought. Forensic metal failure analysis is very established, any decent city has at least a few labs. Send it out, have them throw it under the SEM, x-ray, chem analysis.

    • i agree with uncommon sense; the repeated 21.6″ blowout means that resonance is at work, however, i believe that something else is responsible for the pressure spike.

      the pressure spike, whatever the cause, begins the resonance pulse which then culminates at 21.6″ , by then is so violent- that the barrel simply cannot withstand it.

  5. It’s all cool sciencey stuff right up until I blast the front of my barrel off. I’m hoping that was happening with weird custom hand loads, not the sort of thing that will happen when using published reload data

  6. The old copper crusher test wouldn’t measure peaks, hence the transducer tests of today. I think its just vibration in the barrel that caused it to fall off, caused by powder that is not burning evenly, Randy

  7. I can create roughly identical secondary pressure spikes for you at the 1100 to 1300 microsecond mark. You don’t need any reloading equipment. Just run some Tula .223 or Silver Lake .223 through your NATO 5.56-chambered AR. If you don’t want the secondary spike, run Federal 5.56 or M855. For a highly instrumented study of the same phenomena (with nearly identical pressure-as-tested charts) see If the barrels “cleanly chopped off at 21.6 inches exactly” then the barrel production process and dimensions were flawed alike on each of the barrels. Interaction of casing material, chamber dimensions, and powder types can easily lead (and do) to secondary spikes. Ask your friends: The last five inches of barrel will not fall off at the 1/1100 of a second mark. Laugh.

  8. It is not a second pressure spike. Like previously noted, it is barrel resonance. Similar to a tuning fork. And it is not only chamber pressure, but bullet velocity in the barrel that affect barrel harmonics.

    For an example of how this is positively utilized, see the Browning B.O.S.S. ( ).

    (What is believed to be a second pressure wave is just the reflected initial pressure wave returned to the chamber end at a lower intensity.)

    • Barrel resonance does not cause the chamber to expand, which is the main metric in the post above. Resonance does not register with a strain gauge or a piezo pressure sensor. Barrel resonance (and its affect on accuracy) is simply a different (though interesting) phenomena.

      • When a cartridge ignites, it creates an accoustic and pressure wave that travel down the barrel. It reaches the muzzle of the barrel and is reflected back to the chamber. If the rate of increase of the pressure wave is close to the resonant frequency of the barrel, a lower energy pressure will be recorded as a second spike. The pressure gauges can’t tell the difference between the the waves. They just report how the material is acting.

        The barrel diameter or barrel length at 21.6 inches is why the barrel breaks at that point.

        The math is similar to using the fourier series and oscillations inside waveguides.

        • It is not Fourier but Bernouilli analysis that describes the wave behavior. (College was a long time ago…)

  9. Good thing I just read this post. My phaser has been overheating lately and when it crosses with my light saber beam across the holodeck, well…all hell breaks loose. I was going to try tuning the subspace warp coil but … oh well, I’ll fiddle with it next week. Thank again for the info.

  10. Contact some experts in the silencer industry like AAC and Silencerco. This is right up their alley. And probably already have the answers.

  11. Sorry, gang.

    A barrel “lopped off” due to a pressure spike is something I would have to SEE in person.

    A couple gazillion bullets fired annually for how long now and all of a sudden the barrel can fall in half due to factory ammo?


  12. This subject was more than 10 years old in 2013 and the article is not complete. To see the original article go to

    It was first possible to see secondary pressure events with introduction of the first RSI PressureTrace system, now nearly 15 years ago. Commercial SAAMI piezo testing labs rarely, if ever, capture pressure curves. The phenomenon is not likely to occur in a short, tight, universal receiver barrel.

    For several years numerous threads concerning the phenomenon were on the accurate reloading forums. Denton Bramwell, consultant to Aliiant’s Lake City plant and PressureTrace user was a regular contributor to the forums and a proponent of the water vapor theory. Denton’s attempts to prove his theory by injecting inert gas into the bore before firing did not work. The best explanation is still the one outlined in the above original technical article on the RSI web site.

    Is it real? You bet, as Charley Sisk has show more than a dozen times. Can it be explained via empirical data, controlled and eliminated? Yes as well.

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