The first propellant in wide use was black powder, a combination of potassium nitrate (saltpeter), charcoal (willow was considered one of the best) and sulfur. Charcoal was the fuel and potassium nitrate the oxidizer. The sulfur worked as a catalyst. A ‘low’ explosive, black powder deflagrates (burns) at subsonic rates where as high explosives detonate or burn at supersonic speeds. To work as a propellant for firearms the propellant must convert a solid (generally) into expanding gas with the rise in gas pressure pushing the projectile (bullet) out the barrel. The more complete the combustion, the more gas and thus higher the velocity. The problem with black powder is that when burnt with only about 40% – 45% converting to gas, the majority is left in the barrel (fouling) while the rest leaves the barrel as smoke. Because of this, black powder works but is not the ideal propellant. Black powder fouling is hydroscopic, meaning it absorbs moisture in the air. It therefore becomes corrosive and can rust out a gun rather quickly if the gun is not cleaned after firing. After so many shots it plugs up a barrel, not good in a combat situation. All that smoke gives away the shooter’s position as well, but smoke can also make it hard for follow-up shots as the smoke obliterates your vision.
From Black Powder to Minie Bullet
The downside is the efficiency. Because only about 40% of the mass is converted to gas, the black powder weapon is limited to about 1500 fps or less. At the outside 2000 fps is just about the absolute cap for velocity. Since you can’t push it (the bullet) faster you must push more bullet. Round projectiles limit the amount or mass of the bullet, therefore to increase the energy of delivery, the caliber went up. Some to near .70 caliber and a few beyond. Then came the Minié bullet, and elongated projectile that could grow longer and add more mass per caliber. Bullets came down to around .50 caliber with .50 and .58 caliber bullets dominate to the present black powder market.
Single, Double & Triple Base Propellant
Then in the late 1880’s came nitrocellulose, at first ‘gun cotton’. If nitrated cellulose like wood, cotton or other organics like straw or paper you make nitrocellulose. Straight nitrocellulose is called single base propellant while a touch of nitroglycerine makes a double base powder. Triple base powder introduced in the 1930’s added nitroguanidine, but these are generally only found in large caliber, i.e. artillery and naval guns. Triple base reduced flash and flame temperature but produce more smoke. Early film stocks where nitrocellulose, hence the number fires involving motion picture film and negatives.
Powder Grain Burn Rate
Burning rate is regulated by shape of the powder grain. Ball powders are self-explanatory, flake powders are ball powder flattened. A graphite coating also protects the grains prevents them sticking together and dissipates static electricity. Most ball and flake powders are single base powders. These all burn from the outside, with a diminishing surface area and less gas production, many handgun cartridges and shotgun shells are loaded with these powders. The also burn relatively fast with their high surface area. Whereas, extruded powders have a hollow tube-like structure and burn from the inside out as well as outside in and produce a more constant burning rate and gas production. Many of these powders are double base and are use in rifle cartridges. The problem is that single base powders burn rather rapidly, reach maximum pressure and then decline as the powder is consumed and the chamber-barrel volume increases as the bullet progresses down the barrel. Extruded numbers reach a more sustainable, but lower pressure and maintains a steadier ‘push’ creating higher velocities, as the gas pressure remains higher longer as the chamber-barrel volume increases with bullet travel and full consumption of the propellant. Nitrocellulose detonates at 23,950 feet per second, double base powders push that to 25,260 fps. Powders are a chemical cocktail of deterrents, stabilizers, de-coppering additives to prevent copper fouling from shooting jacketed bullets, flash reducers and wear reduction additives to preserve the barrels.