Get out yer Vernier calipers, guys!
The Boys cartridge IS the Browning casing, just with a headspacing belt added and the neck opened out to handle a .55" slug. That's all the difference, yet the Boys' cartridge developed EIGHTY percent MORE power than the Browning.
You can make up the headspacing belt from brass tube, turn it into collars on a lathe, knurl the base of the cartridge slightly and smear in your low-temp silver-solder, then ram the collar down along the casing with a tube for a ram, expanding the collar as you go. When the collar is in the right position, it becomes a BELT. That's when you carefully heat the collar to the 400 degrees F that it takes to make the low-temp silver-solder glue everything together. Now you can finish to precise dimensions.
You can make a bullet-mould and cast your own out of hard wheelweight metal. They won't take the velocity of the originals, but they are a BUNCH cheaper and you can run them fast enough to get .50BMG power levels from.
But it's the same casing. Just like making 8x57 out of 7x57, only adding a belt.
Have fun.