Barrel covers are an important safety device for paintball. They attach to the rear of the marker, with the cover over the end of the barrel. This is to prevent an accidentally fired ball from exiting the barrel and striking someone who may not be wearing the proper gear (like a mask). Barrel covers can also prevent a barrel from being scratched or dinged if they get bumped between games.
Many fields require a barrel cover be used between games, so don't forget to pick one up.