Too late?  
https://www.youtube.com/watch?v=Hl5xbFXrFU4

You know, I wouldn’t mind playing this. Just so long as any romance (and companions) are optional. I prefer to walk the Lonesome Road.

Looks a lot like original Fallout 3’s (now known as Van Buren) title screen.

@Evowizard25  
It’s “Madhotaru”.  
http://derpibooru.org/110292?scope=scpece6e5c44902a8d9cafbd254dfc51355790a5e85c
 
http://derpibooru.org/206452?scope=scpece6e5c44902a8d9cafbd254dfc51355790a5e85c

@Carnath  
Because firing from their mouth is both stupid visually and makes one cringe. Well, me at least.
 
@Detective_Chmilewsky  
Pony hooves seem quite flexible and able to hold certain items. They aren’t as useful as hands, but they can pull a trigger.
 
Not to mention that unicorns have magic
 
Pegasi can manipulate their wings to hold objects {Which they have done in the show}
 
Earth Ponies seems to be able to wield objects quite fine with their tails. Granted, it’d make firing incredibly difficult, but you make do with what you get.

@Detective_Chmilewsky  
Canon has already shown them walking perfectly fine on 3 legs. Also the equestrian ponies hooves are as hard as you think they are. You need to stop comparing them to real world horse hooves.

@Carnath  
It should have some basic realism to it… Firing from the mouth is a bad idea. A 10mm pistol is .40 caliber. That’s quite a lot of recoil for a pony holding it with their teeth (not to mention the neck strain). But I guess whatever works in this instance.

@VermiIIion  
Except how would they pull the trigger with a hard hoof? They also need to walk on their legs, as they aren’t bipedal.

Overmare mentioned in the post where they showed off the gun model that they had to avoid anything that would take more than one hoof, largely due to how combat works in Fallout 3, and in turn Fallout Equestria. That pretty much throws out any idea of using a gun held in a hoof.
 
And if you read through the comments on the site people questioned why not use a hoof based gun. Stability was a big counterargument to that.
 
Two other logical counters to that as well.  
1: They’ll probably make every weapon with noticeable recoil a battle saddle and not a mouth gun, and if it’s a high recoil handgun it will probably be unicorn only because telekinesis.  
2: We’re discussing a post apocalyptic fanfic/video game starring colorful talking ponies, I really don’t think real life physics about a gun’s recoil or the strength of teeth apply here.

@Evowizard25  
Wouldn’t it make the most sense to have hand-guns that have larger trigger-areas so the pony can hold onto it and fire it? Something like this maybe:
 

 
(sorry for my exquisite paint skills)

@Dustcan  
Firing from the mouth? You’d think that they’d invent guns you can fire with your hoof. {I’ve seen some artists come up with different ways ponies can do that.
 
The hoof firing one I believe is best shown by ‘Madhoutu’.

@OneAmongOthers  
Larger guns are strapped to their sides with specialized saddles in the fic. Smaller guns are fired from the mouth.
 
Doesn’t solve the problem completely but hey.

@Background Pony #A96E  
Considering they don’t have hands I would agree, but there is still the problem of reloading and the fact your head is visible when you shoot making the pony easy for a head shot.
 
Best to just figure out a way for them to hold them in their hooves. Firing on the move is pretty much out for ponies unless they are strapped to their sides or something.

@OneAmongOthers  
Makes more sense than using hand-guns.

Their concepts are silly. Did you see that mouth based pistol? Firing that would shatter every tooth in the poor pony’s mouth.

Wasted talent.

@VermiIIion  
There used to be a group that was working on a full conversion mod for FO3 with that purpose. Looks like they decided to start over by just making a new game entirely though. Their site is http://theovermare.com for anyone interested.

@VermiIIion  
Umm.. as far as I know, this game is in fact being developed. Although it’s more like an addon for Fallout 3, if I’m not mistaken.

It would be awesome if there was an actual game of this. I’d play it, even if I haven’t read the fanfics.