Standard 31rd magazine for Tokyo Marui HI-CAPA 5.1. Also suitable for 4.3. Full metal construction for durability. IF you are going to be using this Hi Capa 5.1 in IPSC situations where you drop the magazines onto the ground from height, we recommend you get the rubber bumpers from Nine Ball to protect your magazine butt plate from cracking apart. Check the RELATED PRODUCTS section below for more detail.