The rounds on the right are brass casing too? .
Newbie Questions
1) Why is brass the ideal choice for making casing?
2) Why is lead used in the projectile?
I'll let MilSpec answer in greater detail if he pleases, but just to skim the surface:
1) 'Ideal' depends on your requirements, costs & availability. Casings can be made from brass, copper, steel or aluminium. Brass just happens to be heavily used in the West - in Russia (or USSR), copper was used more. Also, casings needn't completely be made out of one type of material. Many come coated/plated with a different type.
2) It is malleable, and also very dense (therefore more mass = potentially more damage). Again, there is no rule than only lead be used - but due to existing infrastructure and related costs, it is the most practical. But lead is soft so often it is jacketed in a harder material (like steel) in the bullet so it can retain it's properties for longer.
Bullets exist which can have non-lead cores (steel, copper, tungsten among others).