Take one old cartridge and built a new rifle around it. My favourite has always been the old 577 Snider, and despite years of collecting, could not find one that had a worthwhile barrel.
Enter the new "Super Snider", a Martini Henry action with a new Walther 58cal x 30" muzzle loading barrel 1:48" twist and known dimensions of 0.575" and 0.585". Add to that a special bullet mould from CBE called to 480.CCBB (Bumble Bee) and a load of 75gnd Wano 2P to produce outstanding accuracy at 75 yards.
The maximum load tried has been 92gns Wano2P and a custom 620gn for those days when you want a nose bleed!
It's big seen here next to a 50/70 and a 45/70