Can you melt brass, like a hand full of 9mm, and cast them into useable bullets with standard steel molds?
Not likely. The steel mold may take the heat, but it would not be practical. The brass has too high of a melting temperature. You would have to wait a long time for the mold to cool before opening it.
Also, you would have to have some kind of crucible to get the brass hot enough to melt. No home casting outfit intended for lead alloys is ever going to get hot enough to do that; you would need something with a blower on it, fire clay, fire brick lined. The only reason we can use lead for casting is because of its low melting point and its high freezing point...they have to be fairly close to one another to be practical for steel/Iron/brass/aluminum mold casting.