There is no reason why you couldn't as long as the rifling is in good condition. The 505 has a long neck which is a plus when using cast bullets. There aren't many bullet designs for the 505 but there are quite a few for normal 50 cals (510) - these could then be sized down in stages. If it were me and I couldn't find a mould of the right size, weight and shape I would either approach someone to make one (like CBE, Accurate Molds, NEI etc) or have my own GC design using the Mountain Molds site.

Another alternative would be to have Paper Patch mould made up and these offer several advantages. Not that you really need any expansion with a 50 cal, but you could use soft lead with PP mould. These moulds (eg like the ones made by CBE) are adjustable for weight, so you get a lot of optionally in the one mould. The other advantage is you don't need gas checks to achieve high velocities.

Once you are set up it would make for pretty cheap practice.