A few nights ago in a dream I had an epiphany and I believe I created a practical design for a mouth-operated gun as seen in Fallout Equestria. Granted, it isn't described in appearance as it is in the fic, but it's certainly the most effective, as said by some gun engineers that I showed this to.
The concept of it is like how the butt of a human gun transfers recoil into the arm instead of the gun (preventing it from flying away), a pony gun is built to be secured to the jaw via support areas for the palette, chin, teeth, and cheeks an so the recoil is transferred into the using pony's neck. (and let's be clear, a pony's mouth and neck are far stronger than a human's, making this concept realistic)
For a manually set gun such as a revolver, the hammer is set by pressing a plate with the tongue on the front side of the mouth. The trigger is on the roof of the mouth, underneath the palette support, and is set off by pressing the tongue against the roof of the mouth. A semi-auto or full automatic gun (or in some cases, a magical-energy beam weapon) would just have the safety and trigger.
The side flares, or the equivalent of the stock, acts as a side guard to block debris and exhaust, and also to counterbalance the barrel, keeping the center of gravity near the center of the gun so as to keep it pointed straight. The barrel is topped by TWIN sights, because it's centered. A scope would be off center as well, being off to the side. You can also have the gun so it's collapsible for holstering.
This is purely my design, and it just makes more sense than the others that I've seen out there.