I always imagined the actual mechanism being centered right behind the weapon. Seems like this one would need some way to offset the recoil, not to mention the weight of the gun, its self, otherwise a revolver or even a heavy duty pistol would snap a pony's neck witht eh recoil, or at least rip some teeth out. I suppose it could use some sort of magical dampeners or something... Presumably the trigger is worked with the tongue. Must be a lightweight pull. How would one use the iron sights on a weapon like this?
I dont believe its ever explained in the story, do rifle-grip weapons like shotguns and assault rifles use the same sort of mouthgrip? I know heavier weapons use battlesaddles but Pip gets a shotgun in chapter...2 I think, and doesnt even HEAR about battlesaddles till she meets Calamity. If they did use the same mechanism itd be reeeeally awkward.
I guess thats the great thing about designing something for ponies, you can explain a lot of things away with "its magic" XD