Although not his first foray into fiction related to the Toaru Majutsu no Index "universe",
with his short story,
Toaru Bushitsu no Fanatic, translated as A Certain Clubroom's Fanatic appearing in >Pseudepigrapha Railgun
alongside >Toaru Jihanki no Fanfare,
where the characters of Index are treated as fictional characters,
(this story shows that both stories occur in the same universe)
Toaru Bushitsu no Fanatic
A Certain Clubroom's Fanatic
which was written by (non Kamachi Kazuma)>Fushimi Tsukasa
and illustrated by >Ikeda Sakura
FULL RAWS: https://imgur.com/a/uyNp1Lu
English translation: (non js06)
Translated by >Chaos
and edited by >Rohan 123https://www.baka-tsuki.org/project/index.php?title=Ore_no_Im%C5%8Dto_ga_Konna_ni_Kawaii_Wake_ga_Nai:A_Certain_Clubroom%27s_Fanatic:Chapter_1
It was first announced the Machi Asobi event on May 5, 2013, and the first story was published on Dengeki Bunko's page on the Nico Nico service. The first story was published on June 14 with upcoming stories being published every Friday Pacific Standard Time.
After Fushimi Tsukasa finished his half of the story, Kamachi Kazuma then started his half on September 5, 2013, titled Toaru Majutsu no Index Gaiden Toaru Kagaku no Railgun Ore no Mikoto ga Konna ni Abare Mawaru Wake Ga Nai translated as A Certain Magical Index Side Story A Certain Scientific Railgun "My Mikoto Can't Be This Berserk"