With Savage-recommended smokeless propellants, only 250 - 300 grain saboted projectiles are allowed. Whether the bullet is a conventional cup and core jacketed bullet, monolithic copper alloy, swaged lead, or cast lead it makes little difference as long as the appropriate sabot is used. With the typical velocities produced by the 10ML-II, jacketed or copper alloy bullets perform the best on game.
If you want to use cast lead or swaged lead bullets that are bore size (approx. .501 - .503 diameter), you certainly can but not with smokeless. This holds true for other bored-sized or heavier projectiles such as the Hornady .50 caliber 300 grain Interlock FPB #6605 or the Hornady .50 caliber 350 grain Interlock FPB #6600. No smokeless allowed, but Pyrodex P, Pyrodex RS, organic Blackpowder, or Blackhorn 209 both are allowed propellants. For lead bore-sized projectiles, the Hornady 385 grain "Great Plains Conical" (#6620) is another option.
Note that unjacketed lead bullets whether swaged or cast require bullet lube to prevent bore leading. With the Savage 10ML-II's 1:24 twist, good accuracy has been obtained from heavy cast conicals, such as the "White Rifles" type of 460 grain lubed conicals. Not only is lubrication required with heavy lead conicals with velocity must be kept low to prevent the bullets from stripping the rifling. 90 grains by volumetric blackpowder powder measure of Pyrodex "P" is the traditional load for these very heavy conicals. For best results, saboted rounds are suggested rather than bore-sized.
Note that Pyrodex RS, Pyrodex P, and organic FFg Blackpowder are corrosive propellants, so use of heavy conicals (or other projectiles) requires immediate clean-up just as would be the case in any muzzleloader using these caustic propellants.