As for the projectile, I found a few options.
First, I could buy CNC turned solid copper or brass bullets. The problem with those is the price. They end up being $20 dollars per projectile. Twenty fucking dollars.
Second, I could get a copper rod, that would cost me $75 for 3 feet, and machine my own bullets by hand. That's not so bad, I can match them for the bore diameter and everything.
Third, I could get nylon or teflon rod and make sabots for either stock 50 BMG bullets, or straight up 1045 bar stock. Or an old bit of HSS from an endmill shank. Or even carbide.
Fourth, I can swage copper tubing onto a steel rod, and turn them both on the lathe simultaneously to bullet shape, with only the cannelured copper as bearing surface. I'm looking around for the proper copper tubing right now.
I'll ask at work for the nylon or whatever plastic I could use as a sabot, because that might very well be the cheapest option while requiring the least amount of work.
Right now I'm trying to focus on making this as safe as possible, such is the priority. Second is trying to keep it relatively low cost, because I want to put more than ten rounds per year through it without having to sell my kidneys. Third, is some measure of accuracy (car engine block at 50 yards). Fourth is seeing how long the components can last if I keep MV at 3100ft/s or so.