Muskets began replacing harquebuses in the final third of the sixteenth century. However, the change-over was slow. Harquebuses were relatively light weapons, although their effectiveness was limited by their small calibre. Muskets provided the greatest possible penetration, but their large calibre made them too heavy. Around 1590, muskets weighed about 7.5 kg and fired balls about 21.7 mm in diameter. In order to aim them, the barrel, where most of the weight lay, had to be supported by a kind of forked rod, which was a major inconvenience.
The Dutch succeeded in somewhat lightening these miniature cannons. Around 1600, their muskets supported by forks weighed 6 to 6.5 kg and fired balls about 18.5 mm in diameter. Muskets continued to improve, benefiting from technical progress made in the production of barrels. The army of King Gustav Adolf I of Sweden, considered the most innovative in the first third of the seventeenth century, was the first to adopt new, lighter weapons. A chronicler of 1632 reported having seen a company of Swedish soldiers, and among them were "musketeers armed with the new, very light muskets without forks." 
This progress was made possible mostly by modifications to the wooden butt and the barrel. However, the reduction in weight came at the cost of a reduced calibre, and the balls were henceforth only 16 mm in diameter. Finally, around 1650, improvements to musket design produced a weapon weighing only about 4.5 to 5 kg and no longer requiring forks.
Musketeers fought on battlefields in platoons, companies and battalions. Protected by pikemen against cavalry charges, they fired salvos at the rate of about two a minute. That may appear very little compared with archers who could fire numerous arrows in the same amount of time. However, a lifetime of training was needed to become a good archer, while musketeers could learn the basic skills of their trade in a week. As for accuracy of aim, this was not a terribly important concern on the battlefields of Europe in the seventeenth and eighteenth centuries.
With the exception of pikemen, whose numbers continued to diminish, the transition from harquebuses to muskets led to the gradual abandonment of helmets and cuirasses, which were heavy to carry and provided even less protection against gunfire that was growing more and more murderous. The agility required to handle muskets also encouraged changes in musketeers' clothing.