Rifling, by imparting a spin to the projectile as it travels along the
spiral grooves in the bore, permits the use of a long projectile and
ensures its flight point first, with great increase in accuracy. The
longer projectile, being both heavier and more streamlined than round
shot of the same caliber, also has a greater striking energy.
Though Benjamin Robins was probably the first to give sound reasons,
the fact that rifling was helpful had been known a long time. A 1542
barrel at Woolwich has six fine spiral grooves in the bore. Straight
grooving had been applied to small arms as early as 1480, and during
the 1500's straight grooving of musket bores was extensively
practiced. Probably, rifling evolved from the early observation of the
feathers on an arrow--and from the practical results of cutting
channels in a musket, originally to reduce fouling, then because it
was found to improve accuracy of the shot. Rifled small-arm efficiency
was clearly shown at Kings Mountain during the American Revolution.
In spite of earlier experiments, however, it was not until the 1840's
that attempts to rifle cannon could be called successful. In 1846,
Major Cavelli in Italy and Baron Wahrendorff in Germany independently
produced rifled iron breech-loading cannon. The Cavelli gun had two
spiral grooves into which fitted the 1/4-inch projecting lugs of a
long projectile (fig. 12a). Other attempts at what might be called
rifling were Lancaster's elliptical-bore gun and the later development
of a spiraling hexagonal-bore by Joseph Whitworth (fig. 12b). The
English Whitworth was used by Confederate artillery. It was an
efficient piece, though subject to easy fouling that made it
dangerous.
Then, in 1855, England's Lord Armstrong designed a rifled breechloader
that included so many improvements as to be revolutionary. This gun
was rifled with a large number of grooves and fired lead-coated
projectiles. Much of its success, however, was due to the built-up
construction: hoops were shrunk on over the tube, with the fibers of
the metal running in the directions most suitable for strength.
Several United States muzzle-loading rifles of built-up construction
were produced about the same time as the Armstrong and included the
Chambers (1849), the Treadwell (1855), and the well-known Parrott of
1861 (figs. 12e and 13).
The German Krupp rifle had an especially successful breech mechanism.
It was not a built-up gun, but depended on superior crucible steel for
its strength. Cast steel had been tried as a gunmetal during the
sixteenth and seventeenth centuries, but metallurgical knowledge of
the early days could not produce sound castings. Steel was also used
in other mid-nineteenth century rifles, such as the United States
Wiard gun and the British Blakely, with its swollen, cast-iron breech
hoop. Fort Pulaski National Monument, near Savannah, Ga., has a fine
example of a 24-pounder Blakely used by the Confederates in the 1862
defense of the fort.
[Illustration: Figure 12--DEVELOPMENT OF RIFLE PROJECTILES
(1840-1900). a--Cavelli type, b--Whitworth, c--James, d--Hotchkiss,
e--Parrott, f--Copper rotating band type. (Not to scale.)]
The United States began intensive experimentation with rifled cannon
late in the 1850's, and a few rifled pieces were made by the South
Boston Iron Foundry and also by the West Point Foundry at Cold Spring,
N. Y. The first appearance of rifles in any quantity, however, was
near the outset of the 1861 hostilities, when the Federal artillery
was equipped with 300 wrought-iron 3-inch guns (fig. 14e). This
"12-pounder," which fired a 10-pound projectile, was made by wrapping
sheets of boiler iron around a mandrel. The cylinder thus formed was
heated and passed through the rolls for welding, then cooled, bored,
turned, and rifled. It remained in service until about 1900. Another
rifle giving good results was the cast-iron 4-1/2-inch siege gun. This
piece was cast solid, then bored, turned, and rifled. Uncertainty of
strength, a characteristic of cast iron, caused its later abandonment.
[Illustration: Figure 13--PARROTT 10-POUNDER RIFLE (1864).]
The United States rifle that was most effective in siege work was the
invention of Robert P. Parrott. His cast-iron guns (fig. 13), many of
which are seen today in the battlefield parks, are easily recognized
by the heavy wrought-iron jacket reinforcing the breech. The jacket
was made by coiling a bar over the mandrel in a spiral, then hammering
the coils into a welded cylinder. The cylinder was bored and shrunk on
the gun. Parrotts were founded in 10-, 20-, 30-, 60-, 100-, 200-, and
300-pounder calibers, one foundry making 1,700 of them during the
Civil War.
All nations, of course, had large stocks of smoothbores on hand, and
various methods were devised to make rifles out of them. The U. S.
Ordnance Board, for instance, believed the conversion simply involved
cutting grooves in the bore, right at the forts or arsenals where the
guns were. In 1860, half of the United States artillery was scheduled
for conversion. As a result, a number of old smoothbores were rebored
to fire rifle projectiles of the various patents which preceded the
modern copper rotating band (fig. 12c, d, f). Under the James patent
(fig. 12c) the weight of metal thrown by a cannon was virtually
doubled; converted 24-, 32- and 42-pounders fired elongated shot
classed respectively as 48-, 64-, and 84-pound projectiles. After the
siege of Fort Pulaski, Federal Gen. Q. A. Gillmore praised the
84-pounder and declared "no better piece for breaching can be
desired," but experience soon proved the heavier projectiles caused
increased pressures which converted guns could not withstand for long.
The early United States rifles had a muzzle velocity about the same as
the smoothbore, but whereas the round shot of the smoothbore lost
speed so rapidly that at 2,000 yards its striking velocity was only
about a third of the muzzle velocity, the more streamlined rifle
projectile lost speed very slowly. But the rifle had to be served more
carefully than the smoothbore. Rifling grooves were cleaned with a
moist sponge, and sometimes oiled with another sponge. Lead-coated
projectiles like the James, which tended to foul the grooves of the
piece, made it necessary to scrape the rifle grooves after every half
dozen shots, although guns using brass-banded projectiles did not
require the extra operation. With all muzzle-loading rifles, the
projectile had to be pushed close home to the powder charge;
otherwise, the blast would not fully expand its rotating band, the
projectile would not take the grooves, and would "tumble" after
leaving the gun, to the utter loss of range and accuracy.
Incidentally, gunners had to "run out" (push the gun into firing
position) both smoothbore and rifled muzzle-loaders carefully. A
sudden stop might make the shot start forward as much as 2 feet.
When the U. S. Ordnance Board recommended the conversion to rifles, it
also recommended that all large caliber iron guns be manufactured on
the method perfected by Capt. T. J. Rodman, which involved casting the
gun around a water-cooled core. The inner walls of the gun thus
solidified first, were compressed by the contraction of the outer
metal as it cooled down more slowly, and had much greater strength to
resist explosion of the charge. The Rodman smoothbore, founded in 8-,
10-, 15-, and 20-inch calibers, was the best cast-iron ordnance of its
time (fig. 14f). The 20-inch gun, produced in 1864, fired a
1,080-pound shot. The 15-incher was retained in service through the
rest of the century, and these monsters are still to be seen at Fort
McHenry National Monument and Historic Shrine or on the ramparts of
Fort Jefferson, in the national monument of that name, in the Dry
Tortugas Islands. In later years, a number of 10-inch Rodmans were
converted into 8-inch rifles by enlarging the bore and inserting a
grooved steel tube.