I would say the next step is indeed making lost wax molds for the 3D printed parts, and the easiest casting metal other than pot metal would be brass. A lower AR receiver in brass would work. Mix in some other metals and you can get a harder brass. many early rifles and pistols were made from brass.
Why go to brass?
AR receivers (uppers and lowers) are typically made of 7075 aluminum. You could make them out of just about any aluminum and they’d work. The 7075 alloy is just stronger than other aluminum alloys.
The melting point of aluminum alloys that would be useful here (6061 or 7075) are in the range of 900 to 1100+ degrees F. Brass alloys melt up in the high 1600’s to low 1800’s.
What many AR fanboys don’t realize is that, to those of us who work on old-fashioned guns made of steel and wood, AR’s are already effectively made from pot metal and plastic.
AR