There was a fan game that used to be on this site, but I can't find it.
It's a 3D shooter game where you play as Marisa who tries to steal books from Patchouli's library. One day, Patchouli gets fed up and unleashes various deadly traps to try to kill/contain Marisa (or something like that).
I remember there being a loadout option where you can choose between guns and i think customize attachments.
I believe I got that game around 2017 on an older PC.