|M1A destroyed by old '59 German milsurp 7.62x51|
Soldiers in various wars may have used C4 explosives to boil water in the field. C4 burns very hot, and as long as you let it burn out it won't explode, however, the moment when pressure is combined with that burning heat, someone is losing a limb. Smokeless powder will burn at atmospheric pressure quickly, so you can't use it like C4 to boil water. But just like C4, the right combination of heat and pressure will cause it to detonate. Safe loadings are designed to keep smokeless powder from exploding.
|Why you want powder to burn, not detonate|
In the first post in this series we took a look at lead projectiles being fired by black powder. It is not a very efficient explosive as 55% of the combustion products are solids instead of gas. But black powder explodes faster than smokeless powder burns, so in order to harness that energy bore sizes for black powder firearms are large. The bigger an area at the base of the bullet that gas pressure has to push against the more efficiently it can move the projectile down the bore. When a solid turns to gas, pressure is equal on all surfaces that the gas pushes against. So the larger the bullet base, the larger a percentage of total pressure that the bullet can receive at any given time. This is true for air guns, black powder firearms, and smokeless powder firearms. Black powder firearms have to have large bores or the pressure would overcome the strength of the steel and turn the firearm into a bomb.
|Bulky powders with >60% case fill help|
prevent double charges in pistol cases
There are different "grades" of black powder, which have different speeds of detonation based on grain size. Smokeless powder does not come in grades, but in different names with associated uses and burn rates. Compared to smokeless powder, black powder is very uniform. Smokeless powder ranges from ultra fast pistol powders (Norma R1), to powders slow enough to propel 20mm cannon shells (WC872). When the range of ballistic performance is compared, black powder seems very uniform.
Without going into the details of converting a mol of smokeless powder into the equivalent molar mass of gas (I did it once to show the process) the basic volumetric change is you can expect 2.4 liters of gas from a powder charge that goes into the average 308, but at the high temperature that the gas coming off the burning powder it will be over 7 liters in volume due to heat. Think about how much energy is in a Ford 7.3 liter Powerstroke diesel engine. Then imagine every cylinder firing at the exact same time. That is about the best mental comparison I can think of for how much energy is released when a rifle cartridge is touched off, and energy wise it is a poor comparison.
Leaving black powder behind, smokeless powder comes in three main shapes and three types. Flake, stick, and ball shapes, single, double, or triple base as far as chemistry. Single base is nitrocellulose. Double base is nitrocellulose plus nitroglycerin or nitroguanidine. Triple base is all three. The shape of the powder influences things like burn rate, ease of ignition, and potential case fill (there are some powders that are "sponge" shaped by mashing them with salts and then rinsing the salt away during the manufacturing process, but I don't know of any company still using that method).
|Unique pistol/shotgun powder|
|Extruded powder Left, Ball powder Right|
note the denser pack of ball powder
|Stick powder from Australia, |
note the golden hue of the coating
To sum up, your primer and propellent is there to create pressure by turning chemical energy into kinetic energy. Too little is just as bad as too much, and using the wrong burn rate is potentially disastrous. Since Accuracy is the most important aspect of lethality, it is important to have a consistent primer/powder interface that produces a shot to shot uniform pressure curve.
Remember that accuracy is always relative against what you are shooting. A benchrest rifle capable of 1/8 MOA accuracy at 200 yards is more than you need for deer. A 4 MOA service rifle is plenty for harvesting deer out to 200 yards, as long as you can maintain that level of accuracy. Some handloaders will find a very accurate load that is at a low velocity, and choose to push the bullet faster even though it might open groups up from .5 MOA to .9 MOA. Giving the bullet extra energy and momentum is a good thing, as the difference of .4 MOA is largely academic in most hunting/defense situations.