I know that this is 6 years later, but I had come up with a design that instead of holding the gun themselves, they could end up doing something where they have their 2 front hooves in a chariot, and the chariot had guns connected to it. It has a button on it to fire, and then 2 buttons (or more) to switch weapons. Could also have a few buttons for grenades and such. Manufacturing could all be done by unicorns with magic, and anybody can use them. You could even use them in flight, as the chariot is wrapped around the neck (more like a plow with wheels), and possibly midsection harness.
I know that this is 6 years later, but I had come up with a design that instead of holding the gun themselves, they could end up doing something where they have their 2 front hooves in a chariot, and the chariot had guns connected to it. It has a button on it to fire, and then 2 buttons (or more) to switch weapons. Could also have a few buttons for grenades and such. Manufacturing could all be done by unicorns with magic, and anybody can use them. You could even use them in flight, as the chariot is wrapped around the neck (more like a plow with wheels), and possibly midsection harness.