BBs were fed into that smaller barrel underneath the main barrel. You had to slide the lid to the smaller barrel off to the side, pour the bb's into it, and then slide the lid back to closed. A trick my dad taught me was to make a funnel with my hand by grabbing the top of the barrel with just my pinky and hand with the funnel top being between your thumb and index finger. You could pour the bb's into that part and they would funnel down through my hand and into the barrel. That way you didn't have to spill them trying to pour them directly into the system.