Sorry, I probably should have organized my post better, because you missed a few things. I stated that my father shot .45 colts and it probably wasn't a good idea.
Guys shooting the Comanche .45 Colt/.410 are primarily whom I have read of using the .444 marlin as well as a couple H&R Survivor users. Once the .444 cases are fireformed to the chamber they can load in the .452 or .458 lead bullets depending on the groove depth of their gun. The length of the .444 case pretty much fills the chamber, so the bullets are set down slightly on a fiber wad etc. to the depth needed to keep the lead out of the lands. Thus their bullet doesn't jump from a .45 Colt length case. You are probably correct that most use .452 bullets and it would be the rarer case to load up .458.
The first suggestion was to use .410 to .412 bullets, balls would also work as you pointed out, if you were shooting from a normal .410. If you aren't using a plastic wad, or cut the wings off said wad, or use a fiber wad and set them down inside the hull and roll crimp, you would have a slightly heavier slug that could be formed from typical bullets. There are .410 to .412 bullets in the 180 gr. range. Developing a load using these would allow him to hand cast his slug and with some modification could add a larger hollow base if he wanted to reduce the weight. Personally, I think a slow moving 200gr. bullet would be a much better close range deer round than a 90 gr. round ball, but both would work probably.
