In the American Civil War, cannons came in two basic forms: smoothbore artillery firing solid shot, and rifled artillery firing shells which had been worked with grooves to increase their accuracy. Although rifled artillery was clearly superior, it was also newer, much more expensive, and slower to produce. As a result, both sides of the Civil War made widespread use both of older smoothbore cannons and newer rifled cannons.
According to researcher and appraiser Jack Melton, the American government had resisted investing heavily in military research and development prior to the outbreak of the war. Because the Confederacy opened the war with a range of older weapons, which its forces had already possessed or which they looted from federal armouries, they inherited the same limitations. The Union far outnumbered the Confederacy in terms of sheer numbers of artillery, but the South quickly rushed new factories into production and also began acquiring large amounts of arms from Britain. On both sides, moreover, there was a wide range of artillery, from small field pieces to massive siege weapons. Artillery referred to as cannons could fire shot ranging in weight from 6 pounds to 24 pounds.
What quickly became apparent, however, was that the most important distinction was not simply the size of the shell but the means by which it was fired. Traditional artillery pieces were relatively simple: essentially, an explosive charge and shell are inserted into a long cylindrical barrel, and the charge is then triggered to fire the weapon. A great deal of engineering could go into ensuring a close fit between the ammunition and the barrel, but in its basic aspects, artillery remained largely the same for much of the early 19th century. These became known as "smoothbore" cannons, and they continued to make up the majority of artillery in America prior to the outbreak of war.
By 1860, however, European weapons designers had already moved well beyond the smoothbore era. In Britain, the military had realized that the accuracy and usefulness of artillery - as well as small arms - could be greatly increased by marking both the barrel and the projectile with matching, spiralling grooves. Now, instead of simply being projected out of the barrel by the force of the charge, the shell spun along the barrel. The spin allowed much greater accuracy. The new design became known as "rifled" artillery.
From the beginning, the advantages of rifled cannons were clear in combat. Rifled weapons vastly expanded accuracy and range. However, they were also far more difficult and expensive to produce. New forms of ammunition were needed. The barrels had to be manufactured to a high standard, and they had to be strong enough to withstand the increased stress of firing rifled rounds. It was these concerns which had restricted adoption of the new weapons prior to the war. However, because of its superiority in the field, by the end of the Civil War the rifled cannon had become the weapon of choice. Armies even undertook the tedious process of converting some old smoothbore weapons into rifled ones.
As a result of the American experience in the Civil War, by the end of the 19th century smoothbore cannons were widely regarded as obsolete. The next major war to involve extensive trenches, fortifications and artillery bombardments was World War I, and rifled artillery dominated Europe's battlefields during that conflict. Today there has actually been a move back to smoothbore guns in some settings, such as main guns for tanks, but only because military designers have developed fin-stabilized rounds which offer the same benefits which rifled barrels once promised.