IMO you may be giving up too quickly to find a potentially suitable bullet.
The .500 Linebaugh uses bullets in the range of .511 - .512
The Montana Bullet Works sells .500 Linebaugh bullets in the standard sizing options of .510, .511 or .512.
These weigh .450 grains and are made with "medium" cast lead.
"Medium" cast would mean that only a small fraction of the surface of the bullet would need to be in contact with the rifling in order to be engraved enough to not be stripped when fired.
Yes, they may be a little difficult to load, but you should be able to find a way since precision long range ML shooters load jacketed bore size bullets.
Many of them use an adjustable sizing die, but that may not be necessary and not all of them do.
The Hornady FBP bullets are jacketed and are made slightly oversized to be loaded in the muzzle.
The main difference is that they have a hollow base for expansion. --->>> https://www.muzzle-loaders.com/horna...n-bullets.html
There's even a way to try to add a hollow base to bullets by drilling a hole into the base using a drill press.
And by using more than one size drill, you could create a stepped hollow base, whether it was done in 1, 2 or 3 steps.
These .500 Linebaugh bullets may be able to be knurled or rasped to help make loading easier, or to bump them up, or both.
Either way, the Montana Bullets are sold in a quantity as low as 50. --->>> https://www.montanabulletworks.com/p...t-450gr-swc-k/
They can also be ordered with SPG black powder lube.
Check out their bullet information page: --->>> https://www.montanabulletworks.com/b...istol-bullets/
They will even resize your bullets smaller if you ordered the wrong size, but you would be responsible for shipping.
Grizzly Bullets also sells two different .511 diameter bullets.
One weighs .435 grains and has a gas check, the other weighs 525 grains. --->>>https://www.grizzlycartridge.com/sho...-dia-50-count/
Gad's Custom Cartridges sells .50 - 70 bullets with a diameter of .513 @ 450 grains, and also two different bullets with a diameter of .515 that both weigh 350 grains that are labeled the .56 - 50 Original and the .56 - 50 Taylor.
See the cast bullet chart and the corresponding photo of the bullets that's labeled Table 4: --->>> http://gadcustomcartridges.com
It would be nice if you could find a bullet weight range that shoot accurately.