Complex sentence: The HE shell initially developed for the 17-pdr lacked power: due to the high-powered cartridge; the shell walls had to be thicker to stand the stresses of firing, leaving less room for explosive.
Split into simpler sentences: The HE shell initially developed for the 17-pdr lacked power. Due to the high-powered cartridge the shell walls had to be thicker to stand the stresses of firing, leaving less room for explosive.