Bullet Thermodynamics

      The thermodynamics of a gun are unique for a
 couple reasons. One is that it is a combustion process
 that is known to have too little oxygen for complete
 combustion. This is because within the chamber of
 the gun, there is not enough volume to hold the
 propellant and the correct amount of air. For this
 reason, oxidizer has to be included in the propellant.
 Along with the oxidizer, there must be fuel in the
 gunpowder. The fuel comes from charcoal and sulfur.
 The charcoal provides carbon to the reaction, and the
 sulfur provides fuel, as well as lowers the ignition
 temperature of the gun powder. Charcoal (C) has an
 ignition temperature of 350oC, whereas sulfur (S) has
 an ignition temperature of 200oC. The oxidizer used
 in gun powder is potassium nitrate (KNO3), which
 allows the combustion to happen in the absence of
 air. A basic balanced equation is as follows:

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     In order to set off the gun powder, it must be
 ignited. This is the function of the primer. The
 primer contains a small amount of lead styphnate,
 which is an extremely sensitive explosive. It is so
 sensitive that a change in pressure can set it off.
 When the firing pin hits the primer, the lead
 styphnate gets pressed between the anvil and the new
  indentation made by the firing pin. This sends flame
 through the cartridge and ignites the gunpowder.

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     Gunpowder has about 3MJ of energy per kg,
 which comes out to 3000 joules per gram. One gram
 is equal to roughly 15.43 grains, which is the unit
 gunpowder is measured in. A standard .45 ACP
 round has between 4.5 and 6 grains of powder. This
 means a standard .45 ACP load has between 875 and
 1166 joules of energy inside of the casing. Looking
 back at the first section for the kinetic energy of the
 bullet, ~477 joules, we can, roughly, determine the
 efficiency of the system. This gives the system an
 efficiency between 41% and 55%.

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