Speaking as a long time FoE fan- the concept of the mouth grip is, I believe, relatively new- at least in comparison to how long the weapons have "existed" in the story. Especially once fan art and 3D game projects began emerging about FoE, people started creating designs that would seemingly make sense for pony-held firearms. I don't believe it is ever explicitly stated in-story if the guns are built any differently than what we are used to (of course, there is no way for any of the ponies to make the distinction). In which case, it is one of those things that is up to artistic interpretation- short of an official statement from Kkat, there's nothing really saying that one way or the other is the correct way to render the weapons.