Very high quality PEQ-16 style battery box can fit most compact LiPo batteries, and if you want to fit a Small type 8.4v mini NiCD/NiMH type battery, this PEQ comes with an extension part that you can install on your PEQ to make it a bit longer, thereby accommodating the bigger battery. Fits all RIS rails. A great fashion accessory even if you don't need it's battery storage feature.