The first improvement was the replacement of muzzle-loading by breech-loading.
Inventors experimented with breech-loading from the early 19th century CE.
In 1836 CE Johann Nicolaus von Dreyse produced the 'needle gun', so-called because its firing pin was a small needle.
It also brought two other lasting innovations: bolt action to open and close the firing chamber and the use of paper cartridges.
In combat, the needle gun had two important advantages:
the ability to reload it while lying down and a high rate of fire, up to 10 - 12 rounds per minute.
However it suffered from some defects and was outperformed by contemporary rifled muskets.
Of course the design was improved and by the 1860's CE bolt-action breech-loading rifles easily outclassed their predecessors.
In America Benjamin Henry, Christopher Spencer and others invented the repeating rifle,
which had a magazine and could fire several shots without the gunner needing to reload the gun.
The Spencer rifle could achieve rates of fire up to 20 - 30 rounds per minute.
This was so fast that troops who had no experience with repeating rifles tended to waste all their ammunition within several minutes without hitting much.
But well-handled, the increased rate of fire was a significant advantage.
In the 20th century CE rate of fire, range, accuracy and reliability were all further improved, though the basic design of the rifle did not change.
They still have a place among innovations like submachine guns and assault rifles.
War Matrix - Rifle
Geopolitical Race 1830 CE - 1880 CE, Weapons and technology