Then came the bolt action rifle. This was not as quick, as the lever action, in loading another round, but had a better gas sealing mechanism and could fire a more powerful cartridge. It was also somewhat easier to use in the prone position and was easier to maintain. The lever action mechanism was more delicate, although not by much. The generals became enthralled with the possibilities of longer range fire using the bolt action rifles. Thus, despite its popularity with the troops and battlefield effectiveness, the lever action rifle never caught on big time with the military. When World War I came along, a vast amount of combat experience demonstrated that rapid fire was more effective, and very long range fire was rare. Thus the first automatic and semi-automatic rifles appeared before the end of World War I.
During the World War II, the modern assault rifle (the German SG-43) appeared. This weapon fired a cartridge that was not "full power" (it was more like the old lever action rifle cartridge) and could fire single shot or full automatic. Russia introduced their AK-47 in the late 1940s and the U.S. came up with the M-16 (a 1950s development by an American inventor) in the 1960s. This weapon used the higher velocity (and more accurate) civilian .223 (5.56mm) round. The Germans and Russians had adopted a lower powered, but larger, round (7.92mm and 7.62mm respectively) that was heavier and less accurate than the M-16 cartridge. The M-16 design and the 5.56mm bullet won out. But this combination was so effective that no one has yet been able to improve on it.
The U.S. is proposing a new weapon, OICW, that piggybacks a M-16 with a 20mm gun that fires computer controlled shells that explode over the target (showering troops in a trench or around a corner with fragments.) Currently, the OICW weighs three times as much as the M-16 (or twice as much as the many M-16's equipped with a 40mm grenade launcher.) Troops with combat experience don't see the OICW as a replacement for the M-16, but as another special weapon which, if it works in action, will be another useful, but specialized, battlefield tool. What the OICW developers lost sight of was the major attraction (to the troops) of the M-16; light weight. Not just the weapon itself, but also the ammunition. While lots of automatic firepower was preferred for the ill-trained, conscript armies of World War I and II, after 1945, quality troops became the custom. And these guys spent enough time at the rifle range to make every bullet count. Automatic fire was used only in dire situations. An M-16 with a 30 round magazine, and another ten or so magazines carried along, made for a very deadly soldier. So far, it's been a tough act to follow.
The basic infantry weapon went through tremendous changes for about a hundred years, from the mid 19th to the mid 20th century. But design of a new weapon has been stalled for the last half century. In the 1850s, the rifled musket was developed. Using conical bullet with a hollowed out base, this muzzle loaded weapon had a rifled barrel. When fired, the propellant (black powder) explosion hit hollowed out base of the bullet and forced a tight seal with the rifled barrel as the spinning bullet exited the barrel. The tighter seal and spin made for a longer range, more accurate and harder hitting bullet. Now infantry could hit and kill troops a thousand yards away. The battlefield was dramatically changed. Within the next two decades, metallic cartridges became cheap, effective and nearly universal. At the same time, the lever action repeater rifle was invented, making rapid fire possible for the first time. There was one drawback to this, however, the lever action rifles had to use a lower power cartridge. This was because the lever action design made it difficult to get a tight gas seal and a lower power cartridge had to be used. Also, because the cartridges were loaded in a tube under the barrel, they had to use round nosed bullets to prevent a more aerodynamic pointed bullets from acting as a firing pin and setting off a bullet in front of it in the tube. Initially, the lever action was not popular with the generals because of fears that troops would fire off their ammunition too quickly. This was despite the fact that lever action rifles had a devastating effect in combat, and the troops had a tremendous incentive to conserve their ammunition.