This engraving shows a 12-pounder U.S. shrapnel shell c. 1865. It is fitted with a Borman fuze. In the cutaway view, the dark grey is the wall of the shell, the medium grey is sulphur resin, the light grey are the musket balls, and the black is the bursting charge.
In 1784, Lieutenant Shrapnel of the Royal Artillery began developing an anti-personnel weapon. At the time artillery could use "canister shot" to defend themselves from infantry or cavalry attack, which involved loading a tin or canvas container filled with small iron or lead balls instead of the usual cannonball. When fired, the container burst open during passage through the bore or at the muzzle, giving the effect of an oversized shotgun shell. At ranges of up to 300 m canister shot was still highly lethal, though at this range the shots’ density was much lower, making a hit on a human body less likely. At longer ranges, solid shot or the common shell — a hollow cast-iron sphere filled with black powder — was used, although with more of a concussive than a fragmentation effect, as the pieces of the shell were very large and sparse in number.
Shrapnel's innovation was to combine the multi-projectile shotgun effect of canister shot, with a time fuze to open the canister and disperse the bullets it contained at some distance along the canister's trajectory from the gun. His shell was a hollow cast-iron sphere filled with a mixture of balls and powder, with a crude time fuze. If the fuze was set correctly then the shell would break open, either in front or above the intended human objective, releasing its contents (of musket balls). The shrapnel balls would carry on with the "remaining velocity" of the shell. In addition to a denser pattern of musket balls, the retained velocity could be higher as well, since the shrapnel shell as a whole would likely have a higher ballistic coefficient than the individual musket balls (see external ballistics).
The explosive charge in the shell was to be just enough to break the casing rather than scatter the shot in all directions. As such his invention increased the effective range of canister shot from 300 metres (980 ft) to about 1,100 metres (3,600 ft).
He called his device 'spherical case shot', but in time it came to be called after him; a nomenclature formalised in 1852 by the British Government.
Initial designs suffered from the potentially catastrophic problem that friction between the shot and black powder during the high acceleration down the gun bore could sometimes cause premature ignition of the powder. Various solutions were tried, with limited if any success. However, in 1852 Colonel Boxer proposed using a diaphragm to separate the bullets from the bursting charge; this proved successful and was adopted the following year. As a buffer to prevent lead shot deforming, a resin was used as a packing material between the shot. A useful side effect of using the resin was that the combustion also gave a visual reference upon the shell bursting, as the resin shattered into a cloud of dust.