pounds per
square inch.

Briefly the modern gun is a built-up piece, constructed by fitting or
shrinking super-imposed hoops or cylinders one over the other in size
and number as diagramatically explained in Fig. 25, sufficient to
re-inforce the bore to withstand the varied pressures.

[Illustration: Fig. 25. 13-inch B.L.R. (Total Length, 40 feet.)]

[Illustration: Fig. 26]

The twist or rotary motion is imparted to the projectile by means of
the "rifling" in the bore. Fig. 26 shows the cross-section of an 8-inch
gun with the dimensions of the rifling, which is composed of two
elements, the "groove" or spiral cut made in the bore and the "land" or
space between two adjacent grooves. To take these grooves "rotating
bands" of soft metal, generally copper, are fitted to the projectile as
will be explained under "Manufacture of Projectiles."

Classification of Projectiles

Projectiles are classified according to their calibre, type of gun for
which they are intended, material of which they are made, etc., as
per the following scheme used in the U.S. Army for marking cases of
projectiles:

{ Cast steel
Inch { Rifle { Cast iron
{ Mortar { Common steel { Shot
{ Armor piercing { Shell
{ Rendable A.P.
Weight empty--Lbs.
Capped or uncapped
or
Capped and grooved for base cover
or
Uncapped and grooved for base cover
Base } Fuze
Point }

Manufacture of Projectiles

While a high state of development has been attained in the manufacture
of armor-piercing shells attention will be confined to their
manufacture in so much as the methods for improvement hereinafter
suggested are intended to affect the physical and not the chemical
properties of the material, and are, therefore, applicable to all
projectiles in which the stresses to be resisted exceed the resisting
powers of the projectiles as at present manufactured.

The function to be performed by an armor-piercing shell is that of
fully penetrating, without disruption to itself, an armor plate in
thickness equal to, at least, the calibre of the shell in question, and
then be in condition for effective bursting.

The following extracts from the Army and Navy specifications pertain
to:

MATERIAL AND WORKMANSHIP

(Art 20, O. D., U.S.A.)

The projectiles must be of forged steel, sou