Uranium rounds magazine (ingredient for atomic rounds magazine) is hidden because Krastorio 2 seems to remove/replace the recipe. I don't know what weapon the uranium rounds magazine goes with, but Krastorio uses rifle- and pistol-specific magazines instead of generic "rounds" magazines.

edit: Yesterday, I saw that there's a setting to disable the Atomic (and high-yield variant) rounds magazine. So.... I guess this issue is fixed(?)

Sorry for taking a while to respond.
Yeah, I haven't added any support for K2, as I don't really know the mod very well, so have no idea what changes should be done to recipes. Also it adds loads of its own nukes/antimatter weapons, so I would have to do quite a lot of work to support it properly - also I don't really use it, so I keep putting it off.
If you have any suggestions for recipe replacements or suggested yields for K2, I might add support. Also making the graphics match would be a nightmare.

Re:edit - Yeah, those options might help clear things up, so feel free to use them.

I had the same problem with ammo. You can make it craftable by going to the mod folder and prototypes>nukes-californium and changing the uranium-rounds-magazine to uranium-rifle-magazine in the recipe information. This made it craftable + it also works. Don't know if anything else is needed

This should all be fixed in the latest version.