The smoothbore musket was gradually replaced in the service of European and american armies between 1820 or so and the early 1860s. Their immediate replacement was the 'rifle-musket' - in many cases, literally smoothbore muskets that had their barrels rifled and their flintlocks converted to percussion-cap mechanisms. This was due to the advantages of the percussion cap over the flintlock, an the invention of the Minié ball. The Minié ball, despite its name, was a conical lead bullet designed to expand under the pressure of gunpowder ignition and grip the rifling of a barrel. This allowed the bullet to be smaller than the bore going down the barrel, which made loading far easier, yet still grip rifling when fired, making rifles suddenly nearly/roughly as fast to load as a smoothbore musket.