Certainly sounds interesting to think about.
Matt,
1. First, find a source for brass and primer components. If this can't be done, no use going any further. Having custom made is a possibility, but could get awful expensive quick. But it can be done. If you have brass, I'd suspect the brass would have enough material to machine out and make an adapter for .50BMG primers for sure. Not sure I'd try to reload steel cartridges in this size caliber. Not fun to resize, I'm guessing.
2. Find a press big enough to resize the case. In this case, the Lee Classic Cast and RCBS Rock Chucker are likely big enough to do the job and have the larger size threaded hole to take larger dies. If these two presses are too small, you can use a other types of presses and dies similar to stuff used by bench rest shooters, but on a larger scale size wise.
3. Have custom dies made. I'm sure Lee Precision could handle this job at a pretty reasonable price. If not, there are other folks that do this kind of work.
4. Identify an appropriate powder. I'm guessing off the cuff, but it's likely .50BMG surplus powder should work nicely. That's certainly where I would start.
6. Locate a bullet. This could be tough. You could go custom or you could look into making your own. Again, an expensive option to go custom. That said, there are plenty of folk out there that could have a custom mold for lead bullets made and then add their own jacket. Not sure if you need the jacket, but at that velocity, I'd suspect so.
All in all, unless I had the money and a Rooskie machine gun I badly wanted to shoot, it's not hardly worth it due to costs involved overall. But if I did and could afford it....that's another story.
