Complex sentence: From the AKM he developed a squad automatic weapon variant, known as the RPK (-- Kalashnikov light machine gun), and the PK (-- Kalashnikov machine gun), which used the more powerful 7.62 × 54R cartridge of the Mosin-Nagant rifle.
Split into simpler sentences: From the AKM he developed a squad automatic weapon variant, known as the RPK (-- Kalashnikov light machine gun). He also developed the general-purpose PK machine gun (-- Kalashnikov machine gun), which used the more powerful 7.62 × 54R cartridge of the Mosin-Nagant rifle.