[Pistolero]

Does anyone know why some bullets are smaller at the top and bigger at the bottom?

 

[robocop10mm]

Bottle neck ammo

To make room for more gunpowder to make the bullet go faster. If a .223 was not bottlenecked it would be scarcely better than a .22 magnum.

A side benefit is feeding. If you are chambering a .22 caliber bullet into a .36 caliber hole it goes in easier with fewer malfunctions.