Footnotes on The Sound of Movies
- 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.
- 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 %.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.