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Internal Ballistics; Propellant Burn Rates, Matching Powder to Expansion Ratio & More

As we explore shooting sports we are introduced to various propellants. Propellants, smokeless powder is a nitrated cellulose. Early propellants were heavily based on nitrated cotton, or ‘gun cotton’ used mainly in artillery. Modern powders are single based nitrocellulose while double based powders have a dash of nitroglycerine to up the potential chemical energy. Most powder being a solid propellant incorporates a built-in oxidizer, as a cartridge will function in the vacuum of space or under water. Artillery propellants are triple based adding nitroguanidine to the propellant. These all burn very fast.

How Do Propellants Work?

Propellants work by converting to gas. As we’ve mentioned, black powder was relatively a poor propellant as only about 40 percent is converted to gas, the rest are solids like copious amounts of smoke and fouling deposits in the bore. Not very efficient. The conversion for smokeless powders is much higher. Early smokeless powders were about three times more powerful than black powder. A few terms are in order:
Brisance – is the shattering power of an explosive determined by its detonation pressure. High detonation pressure correlates a high detonation velocity producing the shock wave.
Deflagration – is subsonic combustion and propagates through heat transfer. Firearms propellants fall in this category, from black powder to or smokeless powders. They don’t explode, just burn fast in a confined area.
Detonation – propagates at supersonic velocities via shock waves. High explosives like dynamite, TNT, etc..
Burn rate – is the speed of combustion. Most cartridge propellants are rated at with a relative burn rate compared to other propellants. Thus, powders are rated as fast to slow burning propellants compared to each other.
Expansion ratio – is the ratio of the cartridge volume compared to the cartridge volume plus the bore volume. It is the number of times the gas expands until the bullet leaves the bore.

Why Different Propellant Burn Rates?

In the US a handgun is defined as having a barrel length under 15 inches and no shoulder stock. But most semi-autos have less than 5-inch barrels, while commonly carried defensive revolvers have barrels of 4-6 inches. The powder must complete its work in a comparatively short time and distance. The rifle can use slower burn times because they have a longer barrel with longer expanding volumes. Coatings like graphite are used to control the burn rate, slowing down combustion. This prevents detonation. Other additives stabilize the powder allowing longer storage without deterioration. Potassium chloride can reduce the brightness of the muzzle flash, popular in military and law enforcement ammunition. There are many other additives, though most apply to artillery and cannon ammunition.

Internal Ballistics; Match the Powder to the Expansion Ratio

Peak pressure occurs rapidly and then falls off. It then maintains expansion until falling off when the bullet leaves the bore. It best to match the powder to the expansion ratio, this allows for a longer ‘push’ with higher velocities. Ammunition for handguns must do their work in smaller volumes than a rifle. Also rifles tend to operate at much higher peak pressures than do handguns. 9mm and 45 Auto are both verily mild cartridges compared to the .40 S&W, 10mm or the 357 SIG. Plus P ammo ups the milder calibers in power with attendant higher pressures.
Propellants are at heart of our sport, but probably only of interest to the hand loader, as we have no choices in propellant, nor do really know what they are in factory ammunition.

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