What would you consider the maximum hardness for a full bore bullet?
That's actually a very complex question, Bronko. Talking strictly muzzleloaders, what is your goal? Are you trying to stop gas-cutting of your bullet's base and
resultant leading in the rearward portion of your bore? Are you trying to negate possible nose-droop of your bullet during instantanteous obturation that occurs on firing before the bullet even moves? (Reference: "The Bullet's Flight from Powder to Target" by Dr. Franklin Mann, 1902.) Are you trying to penetrate the body and bones of an African or Asian elephant or an India Gaur, or even Yukon moose, large Bison, Cape Buffalo, Kodiak Bear, etc.? Are you pumping up velocity to achieve faster RPS (revolutions per second of your bullet), since more RPS is needed for longer bullets used in a given twist-rate barrel, as in 1000 yard shooting? Does your "full bore" bullet definition allow use of a paper-patch?
Thankfully, we are not limited like a cartridge shooter to a given length for both powder and wads, and can load bullet protective hard card, felt, Poly or other wads over our powder to protect the bullet base and negate gas cutting, no matter how long the powder column is. So that leaves us with the issue of bullet hardness versus the necessary amount of bullet bump-up (obturation) so that the bullet adequately grips the rifling lands and spins fast enough for stability to any given range.
Also, the heavier your powder charge, the more breech pressure is developed at firing, and this is increased as well as the weight of the projectile goes up. That means you theoretically want a harder bullet to withstand that increased pressure without gas cutting or excess deformation. It also means that you reach a point, with cast bullets, where they become impossible for the average person to swage the bullet to perfectly round, for ultimate accuracy and base-seal at firing. Even a sophisticated caster is going to have a hard time trying to swage a bullet at, and especially harder, than 1:20 tin-to-lead alloy.
Yet, for cartridge bullets, the early ones, such as Winchester's, were 1:16 alloy. That's not really very hard, but it helps the bullet keep it shape (for example, "flat-nosed"), as a better hunting bullet.
On the other hand, pure lead is the easiest to work with as far as achieving good obturation for optimum spin on the bullet via the rifling, but it also deforms badly when it strikes something, limiting penetration.
And for perfect bore fit, a paper-patched bullet of pure lead is great, because it will reliably expand at firing and split the patch so it leaves the bullet instantly at the muzzle. Works, but requires more care with muzzle-loading and a relatively clean bore. Good for target work, tricky for hunting.
This is generalized information for readers of this thread and others, but let us know what your goal is and maybe we can provide more specific info.
Aloha, Ka'imiloa