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Why did assault rifles replace high-powered bolt-action rifles in many armies?

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Semiautomatic pistol — small arm Glock .40-calibre semiautomatic pistol.

Assault rifles replaced high-powered bolt-action rifles because they offer a superior combination of portability, high volume of fire, and effectiveness at modern combat ranges.12

While bolt-action rifles were once the standard, they were limited by their manual operation. In contrast, assault rifles are designed to switch between semiautomatic and fully automatic fire, allowing soldiers to deliver a high volume of fire when needed. They utilize intermediate-power cartridges, which are lighter and produce more manageable recoil than the high-powered ammunition used in older rifles, making automatic fire practical and effective.1

Furthermore, the compact design of assault rifles—including modern "bullpup" configurations—makes them significantly easier to handle than longer, heavier bolt-action or semiautomatic rifles. This portability is essential for mobile infantry, such as troops operating from vehicles or helicopters, and provides a distinct tactical advantage in the close-quarters environments of urban or jungle warfare. By balancing these features, assault rifles became the standard infantry weapon for modern armies, effectively superseding the World War II era designs.12