Handloader- readers have doubtless heard of a term called secondary explosion effect (S.E.E.). It is a theory that attempts to explain the catastrophic failure of some rifles while firing seemingly reasonable handloads or reduced loads using slow-burning powders. Theories have been offered and debated in these pages and elsewhere, but they have been just that, theories, because no one has been able to reproduce effects under laboratory conditions. The purpose here is not to debate S.E.E. but rather to report on a specific incident and the results of tests done to discover the cause of catastrophic failure.
One of the great problems with attempting to theorize on the cause of catastrophic failures is that we must do so after the fact. We have the corpse, usually with some parts missing and must try to figure out what went wrong. Learned theories are offered, sometimes conflicting, and we end up with a bunch of folks shouting in print, 'You're wrong.' "No, you’re wrong." Since the event they're arguing about what without benefit of instrumentation, either one could be right. The events I describe here represent the first instance of an event produced under controlled laboratory conditions and documented on industry standard pressure measuring equipment that provides a plausible explanation offered to explain S.E.E.
The following is simple. It goes all the way back to Shooting 101 where we learned that bore obstructions blow up guns. There are no explosions, no mysterious wave amplifications; it's just a case of several factors, combining in worst case conditions to create a bore obstruction with the bullet.
In early 1989 a major manufacturer began development of a load for the 6.5x55mm Swedish that was to be added to their product line. Development was uneventful and all work was done using the copper crusher pressure measuring system, for there were no standards established for piezo-electric pressure measurement in the 6.5x55mm. The copper crusher method of pressure measurement has been with us for generations, but it is not without its limitations. The results obtained are not true "maximum" pressures, and it provides only a single data point. There is no way that one can deduce what is happening during the period the powder is burning, nor can one see other significant ballistic events.
A quantity of ammunition was loaded using a relatively slow-burning, non-canister propellant with a 140-grain bullet. After load development in ammunition manufacturer’s pressure guns, it is common practice to function test ammunition in a variety of available rifles to ensure satisfactory performance before it is released for sale to the public.
As function testing of the 6.5x55mm ammunition was begun using Swedish Mauser rifles, they noticed some of the same signs of excess pressure every handloader is taught. to look for - flattened primers, enlarged primer pockets and heavy bolt lift. All the ammunition fired in the pressure gun had been perfectly acceptable, but SAAMI test barrels and chambers are made to tightly controlled specifications so the first supposition was that some element within the test gun was contributing to high pressures. Then a "spontaneous disassembly" occurred that destroyed the action but left the barrel undamaged. The bore was clear and showed no bulges. It was immediately identified as a high pressures failure and an investigation was begun. The barrel from the wrecked Mauser action was fitted with a collar that allowed it to be mounted in a universal receiver, and an industry standard conformal piezoelectric transducer was installed. Another test was performed using the Oehler Model 82 piezoelectric pressure measuring system equipped with a trace hold oscilloscope.
(Fig !)
round pressure (psi) velocity (fps)
1 48,820 2,601
2 53,849 2,662
3 57,609 2,708
4 57,999 2,720
5 54,093 2,687
6 58,634 2,731
7 62,150 2,754
8 82,120 2,875
Pressure tests are commonly done with a 10-round string and as you can see from the chart, pressures increased very gradually on rounds I through 4. At the fifth shot, pressure dropped and then continued to increase until, at the eighth shot, pressure, went to 82,120 psi; and the technician wisely stopped the test. The raw data was then used to prepare additional graphs (fig. 1) which show that, after ignition, pressures dropped momentarily to near zero on the graph before beginning to rise again.
To interpret this data we have to first understand the ground rules applicable to pressure testing with conformal transducers. The key term here is 'offset" which relates, primarily, to the specific cartridge and the brass used therein and must be determined for each transducer and lot of brass. The offset is the amount of pressure required to obturate the case to the chamber and begin to exert pressure upon the transducer. In this case the offset was 3,800 psi so when we look at the time/pressure curves produced in the test; we must understand that we are not actually seeing pressures below the level of the offset. There is a distinct dip in the curve, however, shortly after the pressure begins to rise when it drops to a level somewhere at or below the offset pressure. All we can say for sure is that, at this point, the pressure is <3,800 psi. Engineers calculated that for the specific bullet being used it would take pressure of at least 5,000 psi just to keep the bullet moving.
As I said, there are a number of variables at work here, but the main culprit is a very long leade or throat erosion. It takes relatively little pressure to eject the bullet from the cartridge case (de-bullet), which produces a significant increase in volume. Unless the rate of gas production is fast enough to keep up with the increase in volume, pressure must drop. The simple equation is PIVI=P2V' where P = pressure and V = volume. It is helpful in considering the phenomena reported here to view the rifle barrel and chamber as a cylinder whose volume is determined by the position of the bullet at any given point in time. If the bullet is moving, the volume is continuously increasing until the bullet exits the barrel.
If P2 is at or below the pressure required to keep the bullet moving it must stop. Then we run into our old friend inertia. Bodies at rest tend to remain at rest, but all the powder burning behind the resting bullet doesn't know about that. It keeps burning and pressure rises. Sometimes we get lucky and the bullet starts to move and relieve some of that pressure, but in a worst case of a rough bore and/or soft bullet it doesn't, and pressure continues to build until something else lets go. Most of the time this will occur around the primer pocket and gas will be released through the flash hole, but we're talking about events that are taking place quickly (milliseconds); and if pressure rises at a rate faster than it is being relieved, a catastrophic failure is inevitable It has been theorized that many 'accidents" represent a combination of effects which combine, in worst case conditions, to produce a catastrophic failure. Robert Greenleaf (Rifle No. 146) presents convincing evidence to show that conditions rarely remain the same, and the condition of the barrel and throat combined with different bullet characteristics can produce markedly different pressure levels for the same load. This is certainly seen in this data where a series of eight shots of the same ammunition delivered pressures ranging, and steadily increasing, from 48,820 psi up to 82,120 psi, at which point the test was stopped. We can, from looking at this test data, presume that all rounds (except perhaps the first) displayed some degree of temporary bore obstruction, but that the bullet was blown out of the barrel. Fortunately universal receivers are capable of containing considerable pressures, and it is certainly possible that the pressure generated by the last shot would have wrecked a standard rifle.
One factor that cannot be accurately measured with this data is the possible contribution of fouling from the bullet itself. It seems reasonable to assume that some accumulated fouling was blown out on the fourth shot, which accounts for the drop in pressure at shot No. 5.
When the engineers were able to examine and expand the time/pressure curves produced during this test, it became obvious that each shot showed a pronounced drop in pressure very early in the ignition/burning cycle and, on the shot where the pressure reached 82,120 psi, it dropped to the baseline before resuming a climb to the stratosphere. It would be easy to think that the fire went out, but a more reasonable explanation is that the burning rate of the powder became even slower. We know that pressure is a major component of the burning rate of any powder, and it depends upon adequate pressure levels being reached and maintained. In fact, what is shown in this case is that the amount of gas being generated was not sufficient to keep the bullet moving. If pressures drop below some optimum level, burning slows down and is often incomplete. Of course there will always be a quantity of unburned powder from any shot, and this observation has led to some of the conclusions regarding S.E.E.
In order for the pressure to rise to catastrophic proportions some other adverse conditions must also be present. These involve the cartridge case, the bullet, chamber and barrel and need to be discussed individually.
Bullet pull: We know that an adequate amount of tension between the case neck and bullet is a prerequisite for uniform combustion. This term, called bullet pull, is independent of the firearm and is routinely measured in the factories. Crimps may or may not be used to increase bullet pull, but most centerfire rifle cartridges depend primarily on tension between the case and bullet. If you've ever committed the sin of firing a cartridge into which you have neglected to dispense powder, you know that the primer alone is perfectly capable of propelling the bullet several inches down the barrel. Pressure generated by a primer alone can be as much as 4,000 psi in a conventional centerfire rifle cartridge; so it is certainly possible, in a normal round, for the primer impulse alone to be sufficient to get the bullet moving before little if any pressure has been generated by the powder charge.
Chamber: In the area of the case neck there must always be some clearance between the case and the chamber wall, but if this area is too large there is little resistance and the bullet can be released with very little pres sure behind it.
Condition of the barrel and throat: The impact of conditions within the chamber and throat are difficult for the handloader to analyze, and a throat that appears normal under cursory inspection may be revealed to be rough and irregular when seen through a bore scope. Greenleaf's report (Rifle No. 146) details how pressure increased as the number of rounds fired through a test barrel grew larger. This can only be attributable to a deterioration of the throat and leade on that particular barrel. In this instance SAAMI standard barrels were used and showed no irregularities, and it was only when the same ammunition was fired in a 'field' barrel with more generous tolerances and wear in these areas that problems were seen.
Bullet hardness and stiffness: The shape and construction of the specific bullet used can be a major factor in the levels of pressure developed by any given load. Bullets undergo some degree of deformation as they enter the bore, and the force required for them to engrave the rifling and obturate to bore dimensions can vary considerably.
Temperature: We know that pressures tend to increase as the barrel heats up, and a round that produces perfectly normal pressures from a cold barrel might show signs of excess pressure when the barrel is hot.
Work presented here answers questions. Some of the findings support theories offered to explain S.E.E. some don't. We haven't, for example, seen any evidence to indicate that there is ever an explosion, and many authorities doubt that there is. Perhaps what we need is a better name. Taken to its most basic component, what we have is that most fundamental cause of catastrophic failures: a bore obstruction. The difference here is that the offender is the bullet itself effect rather than some external source is both predictable and reproducible in the light of this new evidence, but it is highly dependent upon a combination of factors that produce disastrous results. If one or more is absent, everything will probably turn out fine; but when all come together, pressures rise and, sooner or later, sooner or later, something will fail. While it would appear that slow-burning powders contribute significantly, until now we didn't exactly know what to look for. I think it's at least theoretically possible for a bullet to stop in a barrel if the other conditions are bad enough with propellants other than the slower grades.
Have you ever fired a load that you had used often and suddenly gotten signs of excess pressure such as difficult bolt lift or flattened primers, and then fired another that seemed perfectly normal? I think this happens with some frequency, and our normal recourse is to shrug our shoulders and also be a bright red flag waving in keep on shooting; h6wever, this could front of our nose that is telling us that something is wrong. In the light of these findings, it could be telling us that a bullet did a stutter step before it went on out the barrel. The question then becomes what should we do about it. My first suggestion would be a careful investigation of the condition of the bore, especially the throat or leade to see if there is any erosion or roughness followed by thorough cleaning. A chamber cast might be in order to get precise measurements. If the barrel shows obvious signs of wear or throat erosion, the cure is obviously to replace it or set it back and rechamber. If the barrel appears to be within specifications, however, a change of bullet or propellant may be enough to solve the problem. The importance of this information is that it explains, with laboratory documentation, what can happen when the wheels fall off in the worst way. It seems like such a reasonable answer to many of the mysterious ka-booms that good reloaders have had with good handloads, and it is something we all need to keep in the back of our minds in case we encounter something out of the ordinary. While the data here was generated using the 6.5x55 Swedish cartridge, the observations are not specific to that round. They could occur with almost anything.
