The "Shotgun Shell Cartridge 15R Green" refers to an airsoft accessory, specifically a magazine for airsoft shotguns manufactured by brands like Nuprol and S&T, that holds 15 6mm plastic BBs
 
 
Product Details
Capacity: Each shell holds 15 plastic BBs, functioning as an airsoft magazine. This capacity is consistent across the various brands and color options.
Material: The shells are typically constructed from durable ABS plastic or polymer material.
Color: The specific shell is available in green or OD (Olive Drab) green.
Compatibility: These shells are designed for use with tri-shot and gas-powered airsoft shotguns that use shell magazines, and are compatible with models from manufacturers such as Tokyo Marui, CYMA, Double Eagle, and Secutor.
Usage: They are loaded using an airsoft speed loader by pushing the nozzle into the top hole. They can be quickly swapped during gameplay for minimal downtime.