• HVAC

Pistons & Bullets Part 1; How Does a Gun Fire a Bullet & How Does the Formula for Potential Kinetic Energy Come Into Play?

Ever pondered the fact that a firearm like an automotive engine is a heat engine? A heat engine simply converts heat energy created from chemical energy to mechanical energy. In the car, the engine ingests air containing oxygen, and combines with a hydrocarbon fuel, ignites and uses the resultant heat to create expanding gas that drives the piston down the bore. This results in the piston converting its reciprocation motion to rotary motion via the crankshaft and powering the automobile.

How Does a Gun Fire a Bullet?

A firearm works in a similar manner. You load a cartridge or place powder down the bore, ignite it via primer or flint spark igniting the powder (gun powders all contain oxygen and a fuel in the mixture). This mixture burns, converts to gas and drives the projectile down the barrel (cylinder) converting chemical energy through heat on conversion to an expanding gas that drives the projectile down the bore creating a projectile with kinetic energy, a mechanical phenomena to do work; flight.

History of Bullets

This piston in a firearm is the humble bullet. The first metallic bullets where lead balls. Large lead balls were fired singly as a bullet. Smaller lead balls called shot was used in shotguns. Bullets have many characteristics that allow them to do their job of delivering kinetic energy to the target. Target bullets are made to fly accurately and punch a hole in paper. Hunting bullets “pump” energy into the targets causing tissue and nerve damage plus bleeding to terminate the target. Bullets used in combat are the same, but in war there are conventions or rules that dictate their construction.

Brown & Bess Muskets

The early fuel was black powder, a mixture of charcoal, sulfur and potassium nitrate (saltpeter). Black powder is a chemical propellant and is not very efficient leaving majority of it in the barrel as fouling or expelled as smoke, with only about 40 percent converted to gas. This inefficiency limits the power and the velocity to no more than about a 1700 fps (feet per second). So this explains why the British Brown & Bess musket of the Revolutionary War fired a .69 caliber bullet; the bigger the bullet, the more damage. As the Brown & Bess served the empire for over 100 years, the Brits must have been on to something.

Formula for Potential Kinetic Energy

To increase power in the days of round ball and black powder you had to increase the caliber. Most weapons were limited to a practical velocity of about 1,400 – 1,700 fps or less. The kinetic energy formula is KE = mv2. Mass of course is related to our concept of weight, but technically not the same and is represented by the ‘m’, while the ‘v’ is velocity. A slight increase in velocity is telling because velocity is squared or velocity x velocity. But mass (weight, which is mass in a gravity field) is linear. Double the mass the double the KE, but double the velocity you quadruple the KE or in other words it goes up 4 times more energy than with the original velocity. In the black powder days of African dangerous game hunting 4 gauge shotguns with a bore of 1.052 inches where used to fire lead balls of about 4 oz. to achieve enough energy to bring down cape buffalo, elephant and rhino.

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