I have never tried the Hornady 265 in a muzzle loader, but they sure work well in a .444 Marlin. They were designed specifically for this caliber, since most other .429/.430 bullets are made to expand properly at .44 Mag pistol velocities, and don't penetrate as well at the higher .444 speeds.

Hope they shoot well for you. I haven't so far gotten any .429/.430 bullets to shoot as consistently as the .451/.452 bullets. Used to think it was the thicker sabot, but the 200 Shockwaves have really blown that theory. . . .