This message appears with no other mods besides Krastorio spaced out required mods. More errors get added with any other mods that create new ammunition types.

Factorio version 2.1 Experimental

Failed to load mods: ...l-k2-arsenal__/prototypes/utils/PrototypeSearchUtils.lua:11: [K2 Arsenal] Could not find source rifle magazine for eRisel-high-explosive-anti-materiel-rifle-magazine. Matching uranium ammo: eRisel-uranium-shotgun-shell, explosive-uranium-cannon-shell, uranium-cannon-shell, uranium-rounds-magazine
stack traceback:
[C]: in function 'error'
...l-k2-arsenal__/prototypes/utils/PrototypeSearchUtils.lua:11: in function 'findFirstRaw'
eRisel-k2-arsenal/prototypes/utils/AmmoBuilder.lua:59: in function 'makeRifleMagazine'
...pes/ammo/high-explosive-anti-materiel-rifle-magazine.lua:4: in main chunk
[C]: in function 'require'
eRisel-k2-arsenal/prototypes/Ammo.lua:8: in main chunk
[C]: in function 'require'
eRisel-k2-arsenal/prototypes/Final.lua:1: in main chunk
[C]: in function 'require'
eRisel-k2-arsenal/data-final-fixes.lua:2: in main chunk

Could you please confirm that you have this feature enabled in the settings? https://imgur.com/a/MJ8igVB

I’ve added a check for this feature specifically for such cases; now, if it isn’t enabled, the game will prompt you to turn it on at startup.

It does appear that Realistic Weapons setting in Krastorio is a hard requirement. Game boots up with that enabled (it was disabled previously). With my modset installed it also boots up and loads the world without any errors either.

I mean, yeah? That setting adds extra ammo and ways to deal with Biters to the game. What’s the point of installing my mod, which does the exact same thing at bigger scale, if you haven't enabled that feature in Krastorio itself? It honestly never even crossed my mind that someone would want to do that.