The protagonist of the story, Iron, is just a little filly and the daughter of a rich overmare. When she accidentally ends up outside the stable, she suddenly has to fence for herself. With a gun. The catch? She knows nothing about guns. However, that quickly changes, and this cute little filly becomes a deadly marksman in a thrillery adventure where she has to use her skills not only for her own gain, but also for her friends and family. The story gradually changes to a more character-focused drama as Iron's priorities and outlook change throughout her life, though there's still plenty of adventure to go around, and, more importantly, there's so many heart-wrenching moments it's insane. If you like a good mix of adventure and slice of life, and especially if you like stories that'll make you cry, this one's for you. Also has a lot of interesting worldbuilding and magic lore if you're not sold already.

Summary:
My education was designed to prepare me for the difficult role of overmare.
What it didn’t prepare me for was life outside the stable.
And yet, despite the horrible dangers lurking everywhere, the Wasteland proved itself liberating, invigorating and fun.
If only it had stayed that way.

A slice of life drama in a world where daily life has a pony facing death on the regular.

Story Excerpt:

I woke up on a hard surface, with a soft voice calling me. I’d fallen out of bed again, hadn’t I? Too sleepy, I didn’t bother getting back into bed.

The voice, however, insisted.

“Iron Sonata,” a mare called softly.

Couldn’t it wait? I was a little filly, still growing. I needed my sleep.

“Come on, wake up,” the voice said again.

I groaned. This simple physical act shook my whole brain awake, and I very suddenly remembered where I was, and why I was there. Oh. Oh. Ponyfeathers.

My eyes shot open, and I saw a lifted ventilation grate, with a mare’s head poking through. All the other parts of my brain woke up at the same moment as I shrieked and got to my hooves.

Bad Idea. I hit my horn and head hard on the ceiling, and saw the mare’s expression quickly turn to shock and concern, while my own vision rapidly filled with black.

Tags: [MLP: FiM], [Fallout], [Gore], [Non-con], [Narcotics], [Violence], [Death], [Profanity], [Dark], [Drama], [Tragedy], [Crossover], [Alt. Universe], [OC], [Griffons]
Footnote: It's a FoE story, so I understand if that's not your thing. If you're unsure about the red tags, note that even though they play a significant role in the story, most of it is rather toned down. So much so that the story could probably get away with a Teen rating.

Ooh, that's a neat cover art!