Footnotes on The Sound of Movies

  1. For simplicity the calculations assumed that the bullet's mass was lead. Older style 5.56 mm military bullets or the .223 Remington civilian version are copper jacketed but indeed are mostly lead. Newer style 5.56 military bullets have added a steel penetrater in their tip. However, any sparking caused by either kinetic energy converted into heat or the steel penetrator will be relatively minor compared to an incendiary bullet or a Hollywood flashing bullet.
  2. Gasoline is a mixture of hydrocarbons which are slightly different from one source to another. Its combustion properties are influenced by many characteristics including pressure so its properties can show some variability. We consulted numerous sources and found that most gave explosive limits for gasoline of between 1.4 and 7.6 % in air. However, we found sources with slightly different numbers. These ranged from from a low of 1.2 to a high of 8 %.
  3. Many of our readers have pointed out that high powered sniper rifles can't be disassembled and reassembled without at least some loss of accuracy. Their accuracy tends to be influenced by even slight changes.
  4. A 7.62 Nato rifle has a muzzle velocity of about 774 m/s compared to the speed of sound in air of about 340 m/s. Subsonic ammunition is available for 7.62 Nato rifles but at a significant sacrifice of kinetic energy and range. If concealment were a factor, an assassin would likely be better off using a smaller weapon chambered for a subsonic handgun cartridge, such as a customized submachine gun or specially designed target pistol. Both could be mounted with telescopic sights.
  5. Noise levels vary greatly depending on ammunition type, barrel design and length as well as where and how the noise level is measured. A low powered handgun like the 22 caliber handgun can produce sound levels of 150 dB. 12 gage shotguns and 7.62 Nato rifles can produce sound levels around 170 dB.
  6. A number of knowledgeable readers have written us us that the correct term is suppressor not silencer. The noise from muzzle blasts can be reduced or suppressed but not silenced. We continue to use the term silencer because Hiram Maxim's original invention was called a "silencer" by Maxim himself. Also, the term is far more familiar to the general public than the term suppressor.
  7. According to the League for the Hard of Hearing a whisper is about 30 dB, rainfall about 50 dB and a normal conversation about 60 dB. The "fut" sound of movie silencers seems to fall within this range.
  8. Catastrophic firearm failures are often referred to as kBs, short for kaBooms. They can happen for a variety of reasons including barrel obstructions. The degree of destruction depends on many factors and can be as mild as a bulge in the barrel or a completely demolished firearm. Some examples are as follows: 1) a burst rifle barrel caused by a defective cartridge leaving a bullet in the barrel. 2) various .30 cal machine gun failures caused by obstructions. 3) Even though the barrel of this rifle was not obstructed it illustrates what can happen when a rifle explodes. The rifle blew up due to metallurgical deficiencies which existed for years before the catastrophic failure.