I've been fiddling with .172's lately and .510's, go figure.
I've been toying with various re-loadable micro cartridges. Important attribute being brass easy to find.
When I need a barrel for my project, I'm going to try x-caliber. Last current project is black powder metallic cartridge. Using green mountain but they are very limited in options. X-caliber lists lots of options for a reasonable price.