>>37767800Good questions! To give you a point of reference, swivel guns are more an anti-personnel weapon, with a caliber of around 1 inch of diameter. They're basically right on the boundary between something you can fire while carrying it in your arms, and something you can't.
Good question on the ballast tanks too! First, it's the early days of sealing and riveting technology, so this vessel wouldn't have much capacity for depth at all, I think it would have to stay right under the surface of the water, no more than 2-3 meters. Water pressure increases by one atmosphere, every ten meters, so it quickly becomes structurally dangerous to dive deeper than that. The main blocker for spying applications would be range, I think. Since it is hand-powered and has limited space, its operational range is very limited, so it needs to be deployed from a larger vessel for every mission, some kind of frigate equipped with a crane. While the sub is reasonably stealthy, the mothership wouldn't be, so executing missions deep in busy waters might be difficult.
