Buffalo Arms has a range of moulds and mould weights in .50 cal. - www.buffaloarms.com and check out their mould blocks.

My brother bought the 600gr. cast mould from them for his Sharps .50/Alaskan - impressive mould, beautiful bullets and 1st time out with it, was making 1 1/2" 5 shot groups at 100meters with black powder loads - aperture sights on the M1874 Sharps. In fairness to him, I should note that some of his groups were 10 shot, with 7 or 8 inside an inch, with a couple 'flyers' opening the group to 1 1/2". VERY accurate and well made moulds.