Anatomy of a Kaboom / SEE in .40S&W
The Kaboom is something associated [ in the minds of most people ] either with
- unsupported chambers
- squibs, causing the next round to impact the stationary bullet and blow the gun
- careless reloading practice
The incident here was not a Kaboom, as there was no squib, no case wall failure in an unsupported chamber, but there was a reloading glitch. This was a chamber explosion. It has been referred to as a secondary explosive effect. I'll refer to it from now on as the SEE.
A bit of background on the SEE
We have documented proof, in the form of pictures and articles, both in print and online, that the phenomenon has occured in all makes and caliber of pistol and revolver. The same phenomenon also occurs in rifles.
Historically, the SEE overwhelmingly affects reloaders.
There are several physical causes of the blow-up , but here the conditions are
- unobstructed bores, ie: no squib
- pistols and revolvers, not rifles
- an exploded chamber or cylinder, not a case wall failure in a partially supported chamber
If there is a case of it involving factory ammo and a fully supported pistol chamber, it is attributed to a manufacturing fault in the barrel. I have read on online forums of owners claiming a Kaboom with factory ammo, and in every case they have pistols without fully supported chambers. Those case wall failures do not result in exploded chambers and are not an SEE.
I am also not aware of a SEE in a revolver using factory ammo of the correct rating for the revolver.
The term Factory Ammo as used here, is ammunition provided by a major manufacturer like Hornady, Norma, PMP, etc. This factory ammo is full metal jacket, self defence type ammunition, not cowboy action reloads or similar.
Frames have snapped, barrels have broken at the base, but no cylinder explosion. Yet the SEE occurs amongst the cowboy action pistol shooters. A clue, perhaps, to the root of the problem.
The SEE seems to occur mostly amongst sport shooters who reload for .40S&W. They must be doing something in common.
What these reloads were for
These were specifically for IDPA, SSP division. The required power factor is 125 000.
This means that bullet ( weight in grains ) x ( velocity in feet per second ) >= 125 000.
A mild load of .40S&W far exceeds the required power factor for IDPA SSP of 125 000.
In the case of the 170 grain load with Somchem S121, the loading manual lists the velocity with the minimum powder charge of 5.1 grains as 938 feet per second. This results in a power factor of 159460, which is close to that required of a .45 ACP; 165000. A bit hot for SSP.
In sport shooting, split times and accuracy can be improved with lower power loads, so those with a .40 tend to reload to a lower power factor, enough to clear the 125 000 barrier.
In this instance, the load occupied a small volume in a relatively large capacity case.
So what could be wrong with reduced loads ?
This pistol was loaded with 4.4 grains of S121 and a 160 grain copper metal jacket. That load resulted in ten percent higher power than the required power factor of 125000, using 16 percent less powder than the minimum specified by the manufacturer for a bullet ten grains heavier. A reduced load. This is the reloading glitch that caused the problem.
4.4 grains S121 fits into less than half the case volume after bullet seating. This is illustrated by dispensing 8.8 grains of S121 in a .40S&W, and seating a 160 grain jacketed bullet.
That reduced load of 4.4 grains, when doubled, still permits the proper seating of the 160 grain CMJ. Therefore, the reduced load of 4.4 grains uses less than 50% case capacity after seating.
Case fill is important. Too much powder in a case may result in dangerous pressure, and too little case fill produces erratic ignition. Erratic ignition, if severe enough, is what is thought to cause a SEE.
I believe this case fill is the crux of the matter. At less than 50% case capacity after seating, the entire body of powder in the case may possibly lie below the level of the flash-hole when the primer ignites.
In rifles, all reduced loads use a filler to keep the powder against the primer, precisely to prevent flash-over, a condition where the entire body of powder lies below the flash-hole, and where ignition makes the rifle explode.
Yet handgun reloaders happily reduce their powder charge to lower the power factor, unaware that there could be such a thing as too little powder, the consequences of which make a double charge look like a cake walk.
The reloading manual with the S121 load data warns about excessive pressure by too much powder, and is explicit that maximum velocities cannot be safely exceeded. It makes no mention of possible side effects of too little fast burning powder in a relatively spacious case.
Why Factory Ammo does not SEE
The term Factory Ammo as used here, is ammunition provided by a major manufacturer like Hornady, Norma, PMP, etc. This factory ammo is full metal jacket, self defence type ammunition, not plinking, cowboy action loads or similar.
Factory ammo provides the best performance commercially available for a cartridge, and performance is equated to feet per second and by definition, kinetic energy of the bullet.
Every manufacturer prides itself on the feet per second velocity of it's load, higher being favoured over lower. A few manufacturers are well known for their high velocity ammunition, catering for the self-defence market.
To get optimum performance from a cartridge, case fill is close to 100%, sometimes even a bit higher with compressed loads.
This is why I believe factory ammo does not SEE.
Case fill is 100%, or very close to it. You will never find a factory self defence load with anything close to 50% case fill. An armchair ninja commented that his factory semi wadcutters have 50% case fill. I don't consider that factory ammo, they are just commercial plinking reloads. Moron.
Interestingly, the SEE does not seem to occur in flake powders with lead bullets.
Factory self-defence ammo is high pressure. Recoil can be very stout. Extended shooting with factory ammo may result in loss of skin on your hand. But it does not blow the chamber to pieces.
Factory ammo is concerned with maximum performance, and powder manufacturers provide their data with those maximum loads in mind, and caution not to exceed them.
Powder manufacturers do not market their powders for reduced loading, as the whole market is geared for bigger and faster, not lighter and slower.
The effect of reducing loads with fast burning spherical powder, particularly less than 50% case fill after bullet seating, is not documented in the powder manuals that I have hardcopies of : Hornady, Somchem and Winchester.
Modern pistols are well made, strong, and rated to handle excess pressure. What should happen with an over-pressure round ?
When the ammo is hot, we get massive recoil, blown out or perforated primers, and brass stuck in chambers. In extreme cases, the barrel ruptures.
To imply that a 0.1 grain increase in powder, or a 0.1mm decrease in AOL could cause a SEE, makes my bullsh*t meter twitch. Because if that were true, then we are literally playing Russian roulette with every single reloaded round we put through our guns, and guns should be blowing up with great regularity on ranges everywhere. Which they are not.
To say that a double reduced charge will definitely cause a SEE may appeal to the linear logic process, but does it not take much more than double the reduced powder charge to explode a chamber like a hand-grenade ? Double reduced charges may rupture a barrel, but the chamber needs something more to break it into little pieces.
Given 8.8 grains of the fastest pistol powder in the world, any chemical engineer would be severely challenged to design a way for it to produce enough energy behind a .40 160 grain copper jacketed bullet in an unobstructed bore, to blow up a .40S&W pistol chamber.
I reserve opinion on wether a double reduced charge can destroy a handgun.
Getting to the Point
I do not believe a double reduced charge caused this SEE.
I suspect the pistol equivalent of a flash-over. Which is not supposed to be possible in handgun cases using handgun powders.
On primer ignition, the entire body of powder lies below the flash-hole exposing maximum surface area to the primer ignition. Fast burning powder has small grains, and little retardent.
An undercharge being less than 50% case fill after bullet seating.
This fast burning powder is not the correct powder for the .40S&W reloads to meet IDPA SSP power factor. Simply because case fill at reduced loadings is dangerously low.
The loading manual did not provide any guidelines on percent case fill after seating for a given load. They also do not specify an undercharge to be a hazard. They provide the starting load as a guideline to work up from, but no mention is made of avoiding low case fill. Especially in the very fast burning spherical powder that S121 is.
Mention is made in all the manuals that the specified loads must not be deviated from, they just don't warn specifically about the undercharge. The assumption is that deviating from the load means exceeding the powder charge, not reducing it below the minimum.
For sport shooters that reload, the moral of the story is if you want less recoil, use a smaller caliber. And do not deviate from the powder weight ranges as provided in the loading manual.
A Warning Sign
Many shooters report that factory ammo out of their unsupported chambers does not result in bulged brass, yet that ammo is loaded hotter than sport shooting reloads.
The traditional Kaboom is a ruptured case from an unsupported chamber.
If you experience bulged brass from an unsupported chamber in .40 or .45, using reloads of fast burning spherical powder, try and estimate the case fill after seating, and see if it is below 50%.
That bulged brass could be the early warning signs of erratic ignition due to insufficient powder density in the case, pre-cursor to a full SEE.
In that case test for yourself, replace the fast burning spherical powder with a slower burning flake powder to the same power factor, and see if the bulged brass still occurs.