Canister shot (or case-shot) was a kind of anti-personnel ammunition used in cannons. It was similar to grapeshot in which the canister round's effect is similar to that of a giant shotgun shell. Canister shot has been used since the advent of gunpowder-firing artillery in Western armies; however, canister (or case) shot saw particularly frequent use on land and at sea in the various wars of the 18th and 19th century.
Canister shot consisted of a closed cylindrical metal canister typically filled with round lead or iron balls, normally packed with sawdust to add more solidity to the mass and to prevent the balls from crowding each other when the round was fired. At times when the supply of balls was limited, nails, scrap iron or lead, wire, and other similar metal objects were included. The canister itself was usually made of tin, often dipped in a lacquer of beeswax diluted with turpentine to prevent oxidation and rusting of the metal. Iron was substituted for tin for larger caliber guns. The ends of the canister were closed with wooden or metal disks. Attached to the back of the metal canister was a cloth cartridge bag, which contained the round's gunpowder charge which was used to fire the canister from the gun barrel. A sabot of wood, metal, or similar material was used to help guide the round during firing from the cannon. Various types of canister were devised for specific models of artillery field pieces.
When it was fired, the canister would disintegrate and its shards and projectiles would spread out in a conical formation, causing a wide swath of destruction. It was particularly effective during the Napoleonic Wars and the American Civil War, where massed troops at close range (usually less than 400 yards) could be broken up by artillery batteries firing canister. At times, particularly at very close range, artillery crews would fire extremely lethal "double canister," where two rounds were loaded into the gun tube and fired simultaneously using a single charge. Canister played a key role in dispersing the troops assigned to support Pickett's Charge during the Battle of Gettysburg in July 1863 (see Field Artillery in the American Civil War for more information).
At times, trained artillerists would fire the canister shot towards the ground in front of advancing enemy troops, causing the conical pattern to flatten out as the balls ricocheted and skipped off the terrain. This in effect widened the killing zone. An example of this tactic was at the first day of Gettysburg, where Lt. James Stewart's Battery B, 2nd U.S. Artillery on Seminary Ridge skipped canister shot at Alfred M. Scales's approaching Confederate infantry, breaking up their attack and forcing them to take cover in a depression.
The canister round is also known as a case (hence the alternative name of case shot sometimes used for canister shot) and is still used today in modern artillery, particularly in the main armament of tanks. The effect is to turn a large-calibre gun on an armoured fighting vehicle into a giant shotgun. This can be used against enemy infantry even when in proximity to friendly armoured vehicles. The most recent use has been in the 120mm main gun on the American M1 Abrams tank.