length.

For breaching fortifications, on the other hand, curved fire is
necessary; the escarps of modern fortresses are usually covered from
view by screens of earth or masonry in front, so that the projectiles
must pass over the crest of the screen, and drop sufficiently to strike
the wall about half-way down, that is to say, at an angle of 15 deg. to 20 deg..
To destroy the wall, shell containing large bursting charges of powder
are found to be particularly well adapted. Now it is clear that, for a
shell to drop at an angle of 15 deg. or 20 deg. at the end of a moderate range,
the velocity at starting must be low. Hence, for pieces intended for
breaching no enlarged powder chamber is wanted; the effect on the wall
is due to the shell, which must be made of a shape to hold the most
powder for a given weight; and, therefore, rather short and thick. This
gives us a large bore, which need not be long, as little velocity is
required.

For producing destructive effect among troops, a third kind of
projectile is employed. It is called shrapnel, and it consists of a thin
shell, holding a little powder and a large quantity of bullets. The
powder is ignited by a fuse, which is set to act during flight, or on
graze, when the shell is nearing the object. The explosion bursts the
shell open, and liberates the bullets, which fly forward, actuated by
the velocity of the shell at the moment of bursting. Hence, to render
the bullets effective, a considerable remaining velocity is requisite.
The gun must therefore take a large powder charge, while, as the shell
has to hold as many bullets as possible, the bore must be large enough
to take a short projectile of the given weight. Thus, the proportions
of the shrapnel gun will be intermediate between those of the
armor-piercing gun and the shell gun.