If your friend decides to branch out into bottleneck rifle cartridges, I know a bunch of guys who'd buy 22 High Power, 303 Savage, 250-3000, and perhaps 300 Savage. 300 Savage is fairly widely available already, however.
An easier start would be obscure brass and projectiles. Most people who shoot odd calibers are reloaders.
There should be quite a market for rimfire anything, but that would entail a large startup cost.
