Complex sentence: An advanced design chambered for a high velocity.276 Enfield rimless round which was more powerful than the service-issue.303 cartridge, putting the P'13 into service was rendered impractical by the outbreak of the First World War.
Split into simpler sentences: An advanced designed chamber allowed for a high velocity.276 Enfield rimless round, which was more powerful than the service-issued.303 cartridge. However, the P'13 was rendered impractical by the outbreak of the First World War.