Assuming the barrel is an infinite space, and the marbles are small enough to fit in the gaps from an optimal tennis-ball sphere packing: Not just tennis balls

Assuming the barrel is an infinite space, and the marbles are small enough to fit in the gaps from an optimal tennis-ball sphere packing: Not just tennis balls, because after adding all the tennis balls you can, there's still room for marbles to fill in the gaps Not just marbles, because the optimal sphere packing density is the same whether it's all marbles or all tennis balls. Since all tennis balls wasn't optimal, neither is all marbles. I figure things get interesting once the ratio between the two spheres gets closer to 1. (And checking my work, this argument is described in a fancier sciencey way by https://en.wikipedia.org/wiki/Sphere_packing#Unequal_sphere_packing https://en.wikipedia.org/wiki/Sphere_packing#Unequal_sphere_... )