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This is a cannon shell which explodes on impact, rather than as the result of a burning fuse igniting the bursting charge.
Explosive shells usually rely on the gunner lighting a separate fuse on the shell itself prior to the gun being fired. The fuse burns at a set rate, and explodes after a selected time regardless of whether or not a target has been hit or the enemy is nearby. Incorrectly set fuses can kill friends or gunners, or not explode at all. Explosive shells are also limited to mortars and howitzers which lob their projectiles in a high arc.
The percussion shell, however, is cunningly fashioned to take advantage of the explosive properties of fulminating compounds. These chemicals burst into flame when struck, and this can be used to ignite larger charges of gunpowder. The shock of an impact on a target is transferred to the fulminate, which then ignites the shell's bursting charge. Such shells can be fired from ordinary cannons, directly at an enemy.
Historically, the work of Henri-Joesph Paixhans (1783-1854), who combined the flat-firing cannons and a reliable percussion shell, lead directly to the obsolescence of the wooden warship and the rise of the ironclad.
The last researched shot type, Percussion Shells are available to howitzers and mortars once researched, and replace explosive shells. They most closely resemble Explosive Shells in that they burst, dealing area-of-effect damage. Unlike explosive shells, however, percussion shells always explode only on impact with the ground, making them much more deadly. When compared to Quicklime Shells, percussion shells are best suited to targeting smaller groups of enemies due to their smaller but deadlier area of effect, while quicklime is best used against large clusters.