Wow! I haven't seen any of those for a couple decades.
Back when shooters were limited to military surplus projectiles, before there were Barnes or Hornady match bullets, a couple FCSA competition shooters came up with the idea of swaging magnum cartridge cases into .50 jackets, filling them with powdered lead shot. Used fired primers to fill the hole in the case to keep powder detonation from just blowing all the guts and feathers down the barrel will-nilly.
Performance turned out to be pretty good if the cases were hand selected for weight consistency, lead was consistently poured into them before swaging, etc. Ensuring all the component weights were the same, bullet to bullet. Then, of course, carefully loaded to consistent powder charge weights, cases prepped accordingly, etc. all the standard match load prep that was expected. The bullet "jackets" being magnum cartridge cases had to have their belts turned down on a lathe to get a proper gas seal in the bore. They were a bit over diameter for .510 bores, but not by much.
Not only did they end up with a pretty passable home-made match bullet, but a frangible at that, which had its advantages. You didn't run the risk of a Ball, AP, or solid steel/brass/copper bullet hitting the dirt backstop and just sliding right up and off the backstop into the distance. These bullets would break into pieces on impact with the dirt backstop, so if anything went over the backstop, it was just frag and it didn't go far.
The couple guys doing this sort of thing, won quite a few matches. VERY time consuming, of course. You not only had to spend all the usual time creating your own match rounds, you had to make the bullets up first.
With all the new commercial match bullets coming on the market, process kind of fell out of favor.
Nice little pieces of .50 BMG competition history.