Cartridge pens were a development using actual cases used in firearms -- and those are generally brass (occasionally steel with a brass coating for some military shells) and can be purchased new from reloading supply firms with either a brass or nickel plating surface. The pens are adapted to using those by drilling out the primer pocket and using either an actual bullet, or with some adaptation, the tip from a slimline kit. Plan c is to make your own nib from brass or corian or similar.
Some have used powder coatings to add to the durability and resistance to tarnishing of the case surfaces.
Slimline tips can come in any finish -- but are ball point form. Several have drilled out bullets (I like the all brass bullets) for rollerballs and gel roller style pens.
PSI recently went to the market with a cartridge like pen -- Drop me a PM if you want a few empty cases to learn with and you can make your own with a few slim line kits -- Think the gun metal slimline kits to have the "bullet look" without drilling out bullets or making your own nib.