A shotgun shell is a cartridge for a shotgun. The shell is most often a plastic tube with a brass base though there are entirely metallic shells too. Conventionally loaded within the shell are a number of small lead pellets or, alternatively, a single slug. Plastic shells are typically red, though shells can come in many other colors (including transparent).
Use this tag whenever a shell is visible in the image. They often appear tucked into a bandolier, so consider tagging the two together. Other tags to consider are shell casing if the shell has been fired and is now empty; and casing ejection if the shell is being ejected from the shotgun.