What is the proper way to write the gauge of shotgon or rifle? Is it,
for instance, simply 20-gauge, or do you put a period before the
number? And what specifically does the gauge measure?

mccook --

There is no decimal before the gauge designation of shotguns.  In the
case of the sole apparent exception, the ".410" shotgun, ".410" is not
a gauge designation; it is the bore diameter measurement in inches. 
Here, from a reliable source, is the interesting explanation:

"Shotgun gauges are determined by the number of lead balls of a given
diameter required to make one pound of that size ball. Thus 10 balls
of 10 gauge diameter are required to make one pound of such balls, or
20 balls of 20 gauge diameter are required to make one pound, and so
forth. This is the traditional, and very old, system. The actual
(nominal) bore diameters of the various gauges are as follows: 10
gauge = .775 inch, 12 gauge = .729 inch, 16 gauge = .662 inch, 20
gauge = .615 inch, 28 gauge = .550 inch. The .410 is named for its
nominal bore size, and is not a gauge at all."

Chuck Hawks: Gauges
http://www.chuckhawks.com/intro_gauges.htm
 

Here is an excerpt from a corroborating explanation of this archaic
definition, which emphasizes the counterintuitive fact that the larger
the gauge, the smaller the bore diameter of the shotgun:

"Over the last 200 years or so, shotguns which, with one modern
exception, are classified in "gauges", have come in a variety of bore
sizes from the massive 4 gauge (and larger) "punt guns" used in the
last century by market hunters to fire at large flocks of "sitting
ducks" to the diminutive .22 rimfire shot loads.

"A gauge is a comparatively primitive form of measurement of the
number of pure lead balls fitting the bore size that equal one pound.
For example, a gun in which 12 lead balls that just fit down the
barrel were to weigh one pound, is a 12 gauge. Said differently, the
bigger the gauge number, the smaller the hole because it takes more
balls to weigh one pound."

Shotgun World: Ammo
http://www.shotgunworld.com/ammo_s042002.html 


It is not clear whether you are also asking whether "12 gauge," e.g.,
should be spelled with or without a hyphen.  The answer with regard to
compound adjectives in general is that hyphens should be used for such
compound adjectives when necessary for clarity of expression.  As one
typical style book says:

"Hyphens may be used to avoid ambiguity. Compound adjectives may be
hyphenated to achieve clarity. Ex: personal-computer program; slow
moving-van."
Rutgers University Libraries Style Sheet
http://www.libraries.rutgers.edu/rul/staff/pub_serv/procedures/stylesheet.html
    
For what it's worth, a Justice Department style book says that no
hyphen is needed in the phrase "12 gauge shotgun":

Bureau of Justice Statistics Style Guide (page 28)
http://www.ojp.usdoj.gov/bjs/pub/pdf/bjssg.pdf


Search Strategy:

Two Google searches proved most useful in finding and verifying this information:

"gauge is a measurement of"
://www.google.com/search?sourceid=navclient&ie=UTF-8&q=%22gauge+is+a+measurement+of%22

gauge bore size shotgun
://www.google.com/search?sourceid=navclient&ie=UTF-8&q=gauge+bore+size+shotgun


If any of the above is unclear, please ask for clarification before
rating the answer.


markj-ga

I'm not 100 percent sure how you write out gauge or what not but with
shotguns the lower the number the bigger. 20 is smaller than 12 and
there are also 10 gauge shotguns too. calibur is written out in
inches. most people don't know this but 9mm and .357 and 38 special
almost the same size. 9.mm i think is .356 calibur.

mccook --

Thanks much for the five stars.

markj-ga