Casings, not bullets. In this image, the casing is the big brass part, and the projectile is the smaller part sticking out the end.

Also notice that 30.06 has a large casing. It's a big round.

It was the standard infantry round for the US through 2 world wars, and was by far the most popular hunting round for decades. It was replaced by the military with the adoption of the 7.62 NATO (essentially a .308, but there's more to it than that) with the move to standardization between the allies, and that was replaced as the standard round by the 5.56/.223 with the move to lower caliber, and now this new Sig whatever.

It's a little outmoded these days and lots of manufacturers have moved on to more modern ammo for hunting and precision shooting, but it remains popular simply because of how ubiquitous it was.