Copper would be tough to cast - I assume you mean making something like an FMJ, swaging a lead core into a copper shell - or machining a monolithic copper bullet?
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Machining a monolithic solid, but let me explain. Some processes like swaging are really simply with giant machines but really tough without them. I've devoted a lot of time to pondering how I would go about making bullets if I really needed to, and cast/machined copper solids keep coming back up. The reasoning is based upon three considerations: tools that I can get/use, my aversion to lead, suitability of one process for rifle and pistol and precision. My thought was that casting copper into oversized rods (say 0.5") and then machining it first into a smaller 0.355" or 0.308" rod and thereafter into bullets would be far easier than attempting to press bullet jackets, and FAR more precise with the tools and expertise that I have available. I use FMJ/OTM/HP/AMAX bullets all the time and generally love lead's greater sectional density, but I will absolutely never have the machinery to efficiency swage the giant lead wires or roll, stamp, cup and press the giant copper plates that Hornady or Berger do. I was lucky to get to tour the Hornady factory as a kid and am still struck by the sheer mass of those machines (several of which are needed to complete a bullet).