In the early 1800s, gunsmiths began

In the early 1800s, gunsmiths began to develop a new type of ignition for gunpowder firearms. In 1805, Reverend John Forsyth of Aberdeenshire, Scotland invented the percussion system of ignition, receiving a patent in April 1807. As opposed to sparks being flaked into a small powder charge, the percussion cap contains fulminate of mercury, a chemical compound which explodes when it is struck, hence the term "percussion cap." The percussion cap looked like a tiny "top hat" or cap, thus the name. The cap was made of copper, slightly conical, with a rim or flange at the open end. The interior of the percussion cap had a small quantity of fulminate of mercury and then waterproofed by coating it with shellac varnish. The cap was placed on a nipple with a hole bored through. When a hammer fell onto the cap, the cap would explode when struck with a sharp blow and sent flames into the barrel, igniting the powder charge and expelling the bullet from the gun. The percussion cap would be placed on the cone at the breech (back) end of the gun.

In 1855 the U.S. Army adopted a .58-caliber rifled musket to replace the .69-caliber smoothbore. The new infantry weapon was muzzle loaded, its rifled barrel taking a hollow-based cylindroconical bullet slightly smaller than the diameter of the bore. The loading procedure required the soldier to withdraw a paper cartridge containing powder and bullet from his cartridge box, tear open one end of the cartridge with his teeth, pour the powder into the muzzle, place the bullet in the muzzle, and ram it to the breech using a metal ramrod. The soldier then placed a copper percussion cap on a hollow cone at the breech. To fire the weapon, he cocked the hammer; when he pulled the trigger, the hammer struck the cap and ignited the powder charge in the breech. Each soldier was expected to be capable of loading and firing three aimed shots per minute. Although the maximum range of a rifled musket was sometimes over 1,000 yards, actual fields of fire were often very short, with musketry relying on volume at close range rather than accuracy at long range.

The first American military arm to be mass-produced was the 1861 Springfield Rifled Percussion Musket. The average soldier could load and accurately fire the long-arm three times per minute. The development of a conical projectile that was smaller than the gun barrel, but expanded when fired to meet the gun's rifling made the weapons easier to load, despite repeated firing without cleaning. Over one million Springfields were produced from 1861-1873.

Up to the invention of the percussion cap system, where the ignition charge was self-contained and only required a sharp strike, gunpowder pistols were direct relatives of their rifle parents. With the advent of the percussion system, Samuel Colt used the new ignition to invent the revolver in 1836. The revolver still had to be loaded from cylinder to cylinder, loading powder and ball one at a time.

In the late 1850s, gunsmiths also began to experiment with an entirely self-contained shot. Their goal was to combine powder, bullet, and ignition system into one compact package, called a cartridge. The development of this technology yielded the invention of lever-action rifles in 1860. The Henry Rifle used the lever action and the cartridge technology to allow highly accurate, powerful, rapid-firing guns. The Henry gave birth to the 1873 Winchester, "the gun that won the West." Cartridges were also adopted by many revolver makers, making them easier to reload, fire, and clean.

The end of the 1800s continued the success of the lever action rifle and the revolver, and saw the advent of "smokeless" powder. Even though it is not totally smokeless, it significantly reduced the amount of smoke and fouling produced by firing a cartridge. The new powder was also highly powerful, allowing cartridges to become more powerful than ever.

The Army, aware of the serious deficiencies revealed in the War with Spain and of the rapid technological changes taking place in the methods of warfare, also undertook to modernize its weapons and equipment. Development of high-velocity, low-trajectory, clip-loading rifles capable of delivering a high rate of sustained fire had already made obsolete the Krag-Jörgensen rifle, which the Army had adopted in 1892. In 1903 the Regular Army began equipping its units with the improved bolt-action, magazine-type Springfield rifle, which incorporated the latest changes in weapons technology.