"As a low-velocity firearm, the arquebus was used against enemies who were often partially or fully protected by steel-plate armour. Plate armour worn upon the torso was standard in European combat from about 1400 until the middle of the 17th century. Good suits of plate would usually stop an arquebus ball at long range. It was a common practice to "proof" (test) armour by firing a pistol or arquebus at a new breastplate. The small dent would be circled by engraving to call attention to it. However, at close range, it was possible to pierce even heavy cavalry armour, heavily dependent on the power of the arquebus and the quality of the armour. This led to changes in armour usage, such as the three-quarter plate, and finally the retirement of plate armour from most types of infantry."
Wikipedia, first widespread use of gunpowder weapons.
Poll