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16-gauge : ウィキペディア英語版
Gauge (bore diameter)

The gauge of a firearm is a unit of measurement used to express the diameter of the barrel. Gauge is determined from the weight of a solid sphere of lead that will fit the bore of the firearm, and is expressed as the multiplicative inverse of the sphere's weight as a fraction of a pound (e.g., a th pound ball fits a 12-gauge bore). Thus there are twelve 12-gauge balls per pound (etc.). The term is related to the measurement of cannon, which were also measured by the weight of their iron round shot; an 8 pounder would fire an ball.
Gauge is commonly used today in reference to shotguns, though historically it was also used in large double rifles, which were made in sizes up to 2 bore during their heyday in the 1880s, being originally loaded with black powder cartridges. These very large rifles, sometimes called elephant guns, were intended for use in India and Africa for hunting dangerous game.
Gauge is abbreviated "ga.", "ga", or "G". The space between the number and the abbreviation is often left out, as in "12ga".
==Calculating gauge==
An ''n''-gauge diameter means that a ball of lead (density 11.352 g/cm3 or 6.562 oz/in3) with that diameter has a mass equal to 1/n part of the mass of the international avoirdupois pound (453.59237 grams). Therefore, an ''n''-gauge shotgun or ''n''-bore rifle has a bore diameter (in inches) of approximately
: d_n =(\frac))})}}}
Explanation: 453.59 (the number of grams in 1lb) is divided by ''n'' to find the mass of each hypothetical pure-lead ball. It is then multiplied by 3 and divided by 4 and pi (which is a rearranged volume-of-a-sphere equation) to find the volume when the density is 1g/ml. The density of lead, however, is 11.34g/ml, so the equation is divided by such to correct the density. So far, the described equation would find r3 in cm. To get the diameter in inches, three more steps must be taken following the order of operations. First, we take the cube root of everything to find the radius. Lastly, it is multiplied by 2 to find the diameter, then divided by 2.54 to convert centimeters to inches.
In addition, this complete equation can be simplified quite easily. The first step is to decimalize the outlying fraction and distribute the 3rd root radical to all the inner terms. In addition, instead of dividing by 11.34, it is possible to multiply by its reciprocal, remembering to include the radical, to get this equation.
: d_n =(\frac)}(\frac})
After this, all of the terms can be separated, keeping in mind that 4 and pi are reciprocals. In addition, all reciprocals can be denoted ''x''−1 instead of 1/''x''. Similarly, all cube roots can be denoted ''x''1/3, which means that any cube roots of reciprocals will be ''x''-1/3
: d_n =(1.27^)(3^})(453.59237^})(n^})(4^})(\pi^})(11.34^})
Since everything on the right side of the equation is a number except for ''n''-1/3, the final step is to find the power of all of the numbers (get rid of the exponents), then multiply all terms together. The final equation looks like this:
: d_n=1.67051437n^}
Furthermore, the third equation can be rearranged to solve for ''n'' instead of ''dn''. Every term is taken to the power of three, and every number is divided by itself on both sides of the equation to get:
\frac =\frac)(3)(453.59237)(4^)(\pi^)(11.34^)}
(For clarity's sake, ''n''−1 has been expressed as 1/''n''.)
From here on, ''dn''3 can be moved to the bottom of the fraction and be expressed as ''dn''−3. Afterwards, the reciprocal of the entire equation can be taken, the powers of the numbers can be found, and the resulting numbers can be multiplied together to get:
n=4.661792232d_n^
An alternate equation (result is in cm):
: d_n = \left(\frac^3}\right)^} = 4.2416~\mathrm \times \frac{\sqrt(){n}}
Another source for a gauge size formula can be found in Shotgun shell.
It should be noted that the results of the calculations given above need not be carried out to further than one decimal place, as shotgun and rifle gauges are stated in integers. Furthermore, the density of the lead that was once used as the standard varied, since bullets and slugs are not made of chemically pure lead, but are instead made of lead alloyed with a variety of materials.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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