I wondered why they made barrels in such a different shape. It's not a neat cylinder or garbage can shape.
"Barrels often have a convex shape, bulging at the middle. This constant bulge makes it easier to roll a well-built wooden barrel on its side, changing directions with little friction. It also helps to distribute evenly in the material by making the container more spherical."
I could be wrong but I think the barrel's shape also allowed it to be rocked back and forth to make it easier to stand it up.
Here are the parts of a barrel:
This is an amazing contraption!