In principle, I like the idea of .30 cal for slinging heavy slugs and going long, due to the fact that the diameter lets you get a lot of weight in the same magazine length, and also requires less effort on the gun to push the same weight in smaller calibers. In similar weights you are close or slightly less BC than the heaviest .22 slug, but get a lot more shot count and lower reg pressure due to the diameter advantage.
All that said, .30 still has a serious problem with slug selection. I have a .30 M3 and tried all of the major slug options and the only good one was the FX Hybrids...and they are really good in my gun. In the recent Matt Dubber video talking about Patriot Javelin gen 2 development, he made a comment that testing revealed most of the current slugs in .30 are the wrong diameter for the current FX barrels. This seems to mesh with my trials. With all that in mind I would skip any new guns in .30 for now. You could get a pack of Hybrids and try them through your .30 crown just for fun, though. It might be a solid shooter.
.25 I don't have experience with, but when I look at the selection it is only slightly better than .30. The safe bet seems to be .22 since so much development has been done in that caliber and slug selection is the largest. Power wise, I see crowns shooting 23gr .22 slugs in the 930-950fps range, just to give an idea of the platform capabilities. If you want to shoot heavier weights and higher powers, then you'll probably want something like an Impact. This is especially important if you want to sling the 34-40gr weights at 950fps+ speeds. The South African guys are maxing out their .22 Impacts to get those levels. Reg pressures 170-200bar and hammer weights mandatory.
