Keep in mind that "new" 20MM cases are usually steel, so if you find brass buy as many as you can pay for. if it's even possible to re form steel cases I'm sure it's a lot more work. That was one problem with the .950 JDJ-they started making cases out of 20MM Vulcan brass, then Vulcan cases started being made out of steel.

Of course, the fact that recoil from a 100 pound rifle was knocking some shooters unconscious was a factor as well.