by Bob Henneman
When it is decided to build a battleship
with a new gun design, the preliminary design
specifications for the ship include the main
gun size (14, 15, 16 inches, etc) and the
desired shell weight, plus either the desired
muzzle velocity or range information from
which the muzzle velocity can be worked out.
These are based on some rough calculations,
basically "In order to get through the
thickness of armor we need to defeat at a
given range, the shell must weigh X-lbs and
travel at Y velocity." Depending on
that nation's design philosophy, the specs
might call for a heavier shell (the US for
example) or a higher velocity (Italy for
example). Reasonable limits on gun size must
be maintained, because all large guns began
life as a cast ingot, and there was a limit
as to how large of an ingot you could cast
without sacrificing quality, and a sound
forging cannot be made out of the casting
if the wall thickness and diameter are too
great. Also, the weight of a gun increases
almost in direct proportion to the CUBE of
the gun size, so the practical limits of
turret and ship design, and treaty tonnage
limitations, also work to limit gun size.
For example, the British 16/45 for Lion II
was only 7% greater in barrel size than the
15/45 for Lion I, but it would have weighed
28% more. Generally, a new gun is designed
to solve a problem with a current gun, such
as not enough striking energy leading to
an increase in gun size or powder charge
+ length.
From these original, rough calculations,
designers can figure out how large of a powder
charge is needed, based on existing powder,
and then complex mathematical calculations
can determine how thick the barrel has to
be to withstand this charge, and how long
the barrel must be so that the powder is
done burning just as the shell exits the
barrel. The pressure curve is calculated,
which lets you know how much thinner the
barrel can get you move away from the breach,
to save weight. If the barrel walls are too
thin, the gun will burst, and if they are
too thick the gun will weigh too much to
be usable. If the powder finishes its burn
before the shell exits, energy is lost and
gun performance will be poor. If the powder
does not burn completely before the shell
exits, energy is wasted and gun performance
will be erratic. There is no set formula
for this which covers all time periods, as
acceptable powder charge densities, the burn
rate of powders, the chemical composition
of powders, the amount of the charge's energy
wasted as smoke, and other factors change
as improvements are made over the years.
But as a general rule, guns could become
of a longer caliber as time progressed. In
the case of some weapons, the size was not
increased, but the length was, to take advantage
of a new powder. For example, an improvement
in powder may allow for a greater charge
density, so the same size gun with the same
size powder chamber will have more energy,
and a longer barrel is developed to use this
energy to increase range and striking energy.
That's why you see a 12/35, followed by a
longer 12/40, then a 12/45, only a decade
later.
Once the length is computed, further mathematical
calculations, augmented by past experience,
give such information as the increase in
striking energy over an existing gun, the
rate of wear, etc, allowing small changes
to me made to maximize these factors. With
size, length, and powder pressure set to
optimum, designers can work out the rough
chamber size, based on the allowable limits
of powder loading density, and calculations
are done to work out the effects of different
powders on range and muzzle velocity. Playing
with several mathematical models, each with
different combinations of chamber volume,
weight of charge, muzzle velocity, and maximum
pressure, allows designers to fine-tune the
gun specifications, and choose the best combination.
A test gun is built to verify that the math
was right, and the new gun is ready for production.
If designers changed only one variable over
an existing gun, such as size, length, or
powder charge density, they might skip the
test gun and go straight to production. This
was the case of the British 15/42, which
was really just an enlarged 13.5/45. Skipping
the test gun phase cut nearly a year off
the development time for this weapon.