I wouldn't reduce slow burning rifle powders below listed values, but its an overgeneralization that every load that only partially fills the case will cause detonations

So this wee little load or powder is going to blow my -06 apart?

INTRODUCTION:
In The March 2003 edition of Man/Magnum, Gregor Woods wrote an informative article on the phenomenon of detonation of certain propellant when used in certain quantities in certain cartridges. A letter from a Mr. Miller of Cape town who stated the following, prompted Gregor Woods' article; "I wish to down-load my 270 in the interest of reducing meat bruising and costs (in that low velocities permits the use of much cheaper "conventional' bullets on game, and also save powder, case life and barrel life). I have been warned that reduced loads can cause the powder to detonate (instead of burning), which can blow the gun to bits like a bomb. Is this true, and if so, how can I avoid it?"

In his response Gregor Woods gave an elaborate explanation on the two known theories behind the so-called secondary explosion effect based on the work done (primarily) by the late well-known American gunsmith and wildcatter, Mr P.O.Ackley. These two theories are the overly rapid-ignition theory and the unsymmetrical wave pressure theory.

THE OVERLY RAPID-IGNITION THEORY:
Like the unsymmetrical wave theory, the overly-rapid ignition theory refers to the practice of using greatly reduced loads of slow burning rifle propellant in large capacity cases, especially because excessive airspace exists between the greatly reduced charges of the slow burning propellant and the base of the bullet.

Within this context the overly-rapid ignition theory holds it that if a load is reduced so much that the case is filled to one third or less of its loading capacity, the surface of the powder would lie just below the level of the flash hole when the cartridge is in the horizontal (or firing) position. It is assumed that the primer flame "scoots" or rushes over this entire and broad surface igniting more powder in less time than it would cover if the powder was to burn progressively from the back of the cartridge case with normal loads and loading densities of around 80 to 90 percent. With such high loading densities the flame has to burn through a relatively solid mass of powder in a relatively controlled manner.

P.O.Ackley also mentions in his discussion on reduced charges and the so-called secondary effect that Norma has a different viewpoint on the overly rapid ignition theory.

The Norma version can be more correctly called the delayed ignition of powder when slow burning rifle powder is used in relatively large capacity rifle cases. Nils Kvale explained in the following manner on page eight of the 'Norma Gunbug's Guide'. "Finally, is there a danger in loading too low? Reloader experts claim there is. If a load is reduced so much that the case is filled to a third of its volume, there is a possibility that the primer flash will rush along the surface of the powder, igniting part of it, creating enough pressure to push the bullet into the forcing cone where it comes to a halt, and then, when the ignition has spread to the entire powder charge a few thousands of a second later, the lodged bullet cannot again accelerate fast enough to keep a dangerous pressure from arising. As a matter of fact, guns have blown up under conditions where no other explanation could be found." The merit of Norma's viewpoint on the secondary explosion effect will be brought up later on in this article.

THE UNSYMMETRICAL WAVE PRESSURE THEORY:
Gregor Woods in contrast to P.O.Ackley calls this theory the clashing wave theory. Both descriptions refer to the same phenomenon however. Quite simply it can be described in the following manner: "During the small interval of time when the charge is being ignited, there may be produced in the gun, under certain conditions of loading, abnormally high pressures known as pressure waves. These pressures appear to result from the hurling back and forth of the gas mass between the breechblock and the base of the projectile. Unsymmetrical charges are the cause of wave pressures. With such charges, ignition is not uniform and the charge fails to fill the powder chamber simultaneously in all its parts with a pressure that is uniform and uniformly increasing. Instead, the portion of the charge first ignited may propagate a pressure wave which may be supplemented an instant later by a similar wave propagated by the portion of a charge next ignited, and so on. Certain experiments made to determine the effect of unsymmetrical charges showed that when the charge was placed loose in the chamber, the maximum pressure attained was 34 tons per square inch as compared to 14 tons per square inch produced by a symmetrical charge of the same weight. The wave pressure may be avoided by filling the entire powder chamber instantaneously with a burst of flame that will ignite all portions of charge at the same time." (Handbook for Shooters + Reloaders Vol. 1)

I do think it is reasonable to assume that an unsymmetrical charge weight of propellant within a given case design refers to a relatively large rifle case filled with a charge of slow burning rifle propellant occupying less than 50 % of the loading capacity of the specific cartridge design.

It may also be logical to assume that under such conditions the burning of propellant does not happen in an orderly/progressive manner, but rather in an uncontrolled and disorderly manner mainly because the less than optimal loading density and the large airspace leads to a type of scattered ignition of the propellant and clashing pressure waves. Symmetrical charges of 80% or more would obviously nullify such scattered ignition of propellant.

SOME NEW DEVELOPMENTS.
Ballistic students and experts have all pondered on the phenomenon of secondary explosion effect for years. Then an article appeared in Hand-loader Journal of Ammunition Reloading in June 1997 (number 187). Titled "Mystery Solved", the author Charles L. Petty came up with some pretty convincing evidence on why the secondary explosion effect occurred and in more specific terms why certain combinations of propellant in certain cartridge designs led to the catastrophic failure of guns using such ammunition. The main difference lies in the fact that a major ammunition manufacturer in America were able to reproduce the secondary explosion effect for the first time.

In his article Mr Petty relates on how a (unnamed) major ammunition manufacturer developed a load for the 6.5 X 55 Swedish cartridge using the copper crusher pressure measuring system. A quantity of ammunition was loaded using a quite slow burning (non-cannister) propellant and a 140-grain bullet. After the successful development and testing in the pressure testing system a batch of ammunition was tested using original Swedish Mauser rifles. With these rifles the classic signs of excessive pressures were picked up, like flattened primers, enlarged primer pockets and difficult bolt lift. In the further testing of ammunition a 'spontaneous disassembly' occurred with a Swedish Mauser rifle that totally destroyed the action but left the barrel undamaged.

The barrel of the damaged rifle was fitted to a universal receiver although pressure testing was now done on the more modern piezo-transducer pressure measuring system.

In an eight shot string of 6.5 X 55 ammunition that was fired, pressures gradually soared from 48,820 pounds per square inch to 82,120 pounds per square inch for the eighth shot.

The ballistic engineers in their investigations came up with some very interesting facts. The moment the firing pin hits the primer and the primer flame ignites the propellant, the chemical energy stored in the propellant is converted into heat energy. Hot gases begin to build up behind the bullet and also build up pressure. As the propellant progressively burns, more hot gases develop as well as higher pressures. Upon the initial ignition of propellant and development of hot gases and pressures, the pressure build up is between 5,000 and 10,000 pounds per square inch; typically enough pressure to overcome the friction between bullet and case neck and to get the bullet moving. This movement of the bullet into the barrel throat increases the expansion volume (room) of the hot gases and also lowers pressure slightly.

In the investigation of the eight shots fired with the 6.5 X 55 ammunition a deviation was observed from the initial combustion process as described above. Based upon the pressure figures obtained from the time of ignition to where the bullet has moved out of the case neck, it was determined that a drastic pressure drop occurred at this point. The difference of this pressure drop was that the bullet had for all practical purposes stopped for a fraction of a millisecond after exiting the case neck. The importance of the older 6.5 X 55 comes into play here because of the generous amount or freebore that exists with such rifles. The rather slow burning characteristic of the propellant that was used was also not sufficient to keep the bullet moving.

However as the propellant continued to burn and produce more hot gases and pressures, the pressure curve was simply too high to get the now (again) stationary bullet moving. What is suggested here is the bullet itself becomes a bore obstruction within a time frame and position in the chamber lead area when pressure figures are at their most critical level.

These finding of course confirm Norma's viewpoint on the so-called secondary effect. Other factors may also play a role here as Mr Petty shows in his article. Apart from the effect of a lot of freebore the condition of the bore and throat can also lead to a delay in bullet travel, which under certain circumstances can promote the excessive or catastrophic build-up of pressures. Bullet pull can also be brought into the scenario of extreme pressures. The reason for this is quite simple since an inadequate or too light amount of tension between case neck and bullet can lead to the premature release of the bullet in a rifle which also has a generous amount of freebore.

The 6.5 X 55 Swedish Mauser is not a large capacity case by any ballistic standard. One tends to get confused if we compare the article and findings of Mr petty with the known description of the secondary explosion effect, namely that the use of greatly reduced charges of slow burning propellants in large capacity cases can lead to catastrophic pressures.

The common denominator however is the excessive airspace that exists or which may develop. A bullet that leaves a case neck prematurely and momentarily comes to a halt in a rifle with a lot of freebore (unrifled section of the throat) when used with a rather slow propellant, also creates airspace. This means that the gas build up is too slow behind the bullet and that a 'temporary airspace' is created behind the bullet even if the original charge of propellant was around 80% of the case volume.

In the Ackley tradition one could say that a symmetrical charge of propellant becomes unsymmetrical under such condition and also that unsymmetrical (abnormal) pressures/pressure waves could develop because of the "created airspace".

In conclusion we may ask whether the secondary explosion effect has indeed been solved. The existence of this effect started to be known towards the end of the 1940's in America amongst the hand loading fraternity. Initially it was thought that under some (unknown) circumstances, a half charge plug of slow-burning propellant would be blown forward by the primer spark into the shoulder / neck area where it would wedge, smouldering, until it detonated just like a bomb.

The misnomer of secondary explosionary effect most probably develop from this misconception. Today this secondary explosionary effect is predominantly associated with the spontaneous disassembly of the rifle due to extreme pressures. In this regard it is helpful to view the rifle barrel and chamber as a cylinder whose volume 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 for some reason the bullet is slowed down or momentarily comes to a halt due to insufficient pressure and because not enough gas is initially generated to keep the bullet moving, the scene is set in a ballistic sense for pressures to rise to catastrophic proportions.

For the handloader today it also emphasizes the importance of the burning rate of a propellant being used in relation to the loading capacity of the cartridge being reloaded.

The Norma version can be more correctly called the delayed ignition of powder when slow burning rifle powder is used in relatively large capacity rifle cases

I do think it is reasonable to assume that an unsymmetrical charge weight of propellant within a given case design refers to a relatively large rifle case filled with a charge of slow burning rifle propellant occupying less than 50 % of the loading capacity of the specific cartridge design.

Then an article appeared in Hand-loader Journal of Ammunition Reloading in June 1997 (number 187). Titled "Mystery Solved", the author Charles L. Petty came up with some pretty convincing evidence on why the secondary explosion effect occurred and in more specific terms why certain combinations of propellant in certain cartridge designs led to the catastrophic failure of guns using such ammunition. The main difference lies in the fact that a major ammunition manufacturer in America were able to reproduce the secondary explosion effect for the first time.

In his article Mr Petty relates on how a (unnamed) major ammunition manufacturer developed a load for the 6.5 X 55 Swedish cartridge using the copper crusher pressure measuring system. A quantity of ammunition was loaded using a quite slow burning (non-cannister) propellant and a 140-grain bullet.

Initially it was thought that under some (unknown) circumstances, a half charge plug of slow-burning propellant would be blown forward by the primer spark into the shoulder / neck area where it would wedge, smouldering, until it detonated just like a bomb.

For the handloader today it also emphasizes the importance of the burning rate of a propellant being used in relation to the loading capacity of the cartridge being reloaded.