Anyone here who hasn't read any 40K and wants a feel for it to try writing something I'd recommend the following as a crash-course on the mood and appeal

https://www.youtube.com/watch?v=JjaYW5Cnr5k

https://www.youtube.com/watch?v=cZc6cr6G2E4

https://1d4chan.org/wiki/Ollanius_Pius note that that's id4chan, making it a comedic take on an event that was dead-serious in the lore, but it gets the point across well.

Any number of short stories but particularly Survivor by Steve Parker(read the commisar Yarrick wiki page before reading this, and get the story however you can)

https://www.youtube.com/watch?v=YROOgLD-wtU

https://www.youtube.com/watch?v=piBhBMrDAVA

Anyone interested enough to do some serious long reading real quick, the gaunts ghosts books are probably the best 40K literature, featuring ordinary, non-augmented humans taking on threats they have no logical chance against and suceeding, as well as villains that are mostly average humans but terrifying.

It is seriously, in my opinion, the coolest fictional universes. The Gaunts Ghosts books are my favourite novels period.