Nickel-Plated Brass
Cartridge brass finished with a thin nickel plating that resists corrosion and feeds slickly, though it is harder on resizing dies.
Nickel-plated brass is ordinary cartridge brass coated with a thin, bright layer of nickel. The plating resists corrosion and tarnish, which makes it popular for duty and defensive ammunition that may sit loaded for long periods, and its slick surface tends to chamber and extract smoothly. The harder finish also reduces the chance of a cartridge sticking in a warm chamber.
For the handloader the tradeoff is that nickel is harder than bare brass and more abrasive, so it wears reloading-dies faster and the brittle plating can flake at the case mouth after repeated cycles. Adequate case-lube is still required for sizing, and many reloaders find nickel cases give fewer total firings than plain brass before the necks begin to crack.