From The KillerJim Mod Wiki

KJMod does not contain any built-in guns, it provides guns through official workshop packs that are usually downloaded by you when you first connect to a game (or via the main menu).

There's two benefits here; firstly we can update and add guns without having to push a new version of the game, secondly and the more important benefit is that we can open-source the whole UE4 project. This allows you to take the official guns and alter them, learn how the system works and build new guns on top of them.



Developer Reference