If fired cold, you can make five shots in the first minute.
the light cannon can make 10 shots in the first minute
Was this an intentional ret con, or a mistake? If it was a mistake, which fire rate is correct?
Sorry, I was unclear. A copper-block cannon fired cold has an alpha-strike capacity of 5, and a sustain of 2 shots per minute. Your kettle guns have an alpha-strike of 10 when fired cold, and a sustain of 2 shots per minute.
The overall fire rate is mostly similar due to water-copper having a similar heat exchange rate as opposed to copper-copper, but you do have an advantage in how quickly you can exchange the heat-sink. The issue is, after the water tubes around the barrel evaporate, you still risk melting the barrel with sustained fire.
So, firing speed between the two cannon models is (mostly) the same after the water in the barrel mounts boils off, but the kettle cannon has a substantial advantage in the opening engagement, and the ability to swap heat-sinks efficiently and safely gives a slight advantage in the long term. More importantly is that the reduced weight of the heat-sink makes the cannon alot lighter, which lets the gun operator move the cannon more rapidly, which results in faster and easier aiming.
Without the barrel mounts, the addition of the water tank doesn't do too much for you. It reduces the weight of the individual sinks and makes it easier to slot them in and out, but since you'd have to redesign the barrel and sink mechanism in order to allow circulatory flow, it doesn't make too much of a difference in increasing overall fire rate.
It's late (or early), so it's possible that I'm just being dense.