Hey Steel Horse,
If you’ll bear with my lengthy explanations I can make it plain for all. Brisance actually has two meanings that we would be interested in. The first is the common meaning used with explosives, propellants or other pyrotechnics. Brisance is basically a “rate” metric. It is the rate at which a combustion reaction reaches maximum pressure. If an explosive charge has a sufficiently “high” brisance rate (lower time) then it can create a shock wave which is what you want from a “high explosive” a “low” explosive like BP doesn’t have the shattering effect of a faster explosive because it has a shallow pressure curve and takes longer to make maximum pressure.
As an aside BP also has a lower final pressure and expansion velocity than other industrial or common explosives. This makes it a better propellant than an explosive, just like nitrocellulose powder is a better propellant than an explosive.
The shattering effect is what the first commercial and military applications were looking for, brisance comes from the French word “briser” which means to break or shatter.
We also use the term when it comes to detonation trains, primers, initiators and in our case percussion caps. In this sense it applies to two elements of the combustion process, the amount of burning gas generated and the distance it will travel. So, it is volume and the distance which are usually, but not always interrelated. You can add more of the base priming compound to increase the brisance of a cap or primer, or you can add an extender or booster. The extender or booster may increase the velocity or the time it will burn (in some cases both). If you have an extender in a cartridge primer it may just allow the burning gas to project out further while it is still at the maximum temperature it will attain.
With an initiator train for a warhead we really don’t care if we create excess pressure or debris because we’re getting ready to make a big bang anyway. In the case of precision shooting we usually want to minimize the effects of the primer and control our pressures and gas volumes with the propellant instead of the primer. If you consult a common loading manual you will see differences in pressures with identical loads except for the primers and a warning you will see increased pressures if using certain magnum primers.
I am attaching a portion of a great article German Salazar wrote for the June 2008 issue of Precision Shooting. These shooters are looking to control the most arcane aspect of each and every cartridge so the search for the primers that have the least amount of pressure deviation at the lowest brisance rate that will effectively ignite their powder. I’m including it because the pictures illustrate the differences better than I can explain them. Note these are Bench Rest primers, and look at the ignition and carry differences between these precision primers by manufacturer alone.
Now to address your astute comment about the common usage of “magnum” caps. You are very correct!
The problem powders are not so much Pyrodex but the other modern substitutes. They are more difficult to ignite. Now that being said the Pyrodex pellets are candidates for magnum caps. I have sample cans of the substitutes but I really don’t use them. I get excited when I read about them and upon trying them it’s sort of like “kissing your sister.” So I tend to relay information about ammunition and loads using Gunpowder and not substitutes.
Real Gunpowder is very forgiving and really doesn’t need the “new fangled” inventions, magnum caps or primers to set it off. In fact attenuating the effects back through the flash hole is more of an issue with those of us that want to be competitive in timed shooting events. I will literally use whatever components I have on hand when loading BP cartridges, it’s really not picky. But, I am very picky about my cones and how the caps I have on hand fit on those cones. As I have written, I have had good success with Treso tubes on even muzzleloaders which probably lets the same amount of gas through the flash hole from the cap side (because of the internal geometry) as a standard primer, but then limits the gas flow coming back through the hole from the chamber.
Look at the sequence in my post above, look specifically at frame 2. There is excess gas which can’t make it through the flash hole on even that 138+ year old Colt’s original tube. If it is given a larger primary chamber in the tube to start down then it can still have a sufficient volume to make it into the chamber and set off the powder with gobs of excess gas to spare. There is obviously a lot of extra gas volume from the cap that continues into frame 3. So a smaller hole with a lead in on the cap side will assure enough getting through for ignition while at the same time limiting the orifice size to let gas flow back through the hole.
This is how we are similar to the Bench Rest shooters, we want just enough gas into the chamber to assure consistent ignition. But then the BR shooter are then concerned about it affecting their pressures, we are just concerned about that pressure having a large hole to get back out of. If I get the time I may conduct some photo tests with caps and different tubes. It would be interesting to see the difference in brisance coming out the chamber end of a cylinder with different flash hole sizes and primary tube chamber geometry. Ideally I would take an old or damaged cylinder and face it off leaving only 1/8” or so of chamber to see the gas shape right at the flash hole exit instead of out at the end of the chamber. If you look at the photos above you can see some of the primers have a nice gas head right at the exit, yet they don’t project very far (they have “lower brisance”).
Regards,
Mako
P.S.
On the cross drilling comment: priming compound already has it's own oxidizer and it couldn't entrain any air into the mix quickly enough to have any effect. Primers, propellants and explosives all carry their own oxidizers and contrary to what you see in movies, TV and read in books they will go off in total vacuum sealed or unsealed, also underwater (or in any other environment) if the items which can be affected by moisture don't get wet.