No side-mounted guns, because the recoil would knock the top-heavy thing over.
Now why is Frank driving around the Soudan, firing pneumatic shells into crowds of Mahometan savages? He's exploring. It doesn't go well. He is set upon by desert warriors right away.
Unfortunately, when they get to an outpost, they discover that the Arabs have cannons. The first shot doesn't wreck the Omnibus, but it's unnerving.
I'm sure he was very sad it had come to this.