And another refinement:
Adding two pairs of ramps definitely improves the performance of my cheap watergun. While the 40(step) Bofors is a nice cheap-built standoff gun (can work without power if you have a river or aquifer on a convenient z-level), the water itself doesn't do much damage at about 80k speed. Kills seem to be mostly due to collisions with floor, walls, ceiling and other creatures. I haven't tested the 45(step) ACP (Aquatic Circulating Perforator) against creatures, but the slightly longer acceleration time adds range, increases the expected damage and can be stuffed with extra minecarts for multiplied cadence per barrel. The reload process takes about 14 steps, so the maximum'd be, what, three carts per barrel?
Yup. You need to put rollers on each flat floor tile to prevent newton's-cradle lowered effect (can result in shooting backwards) and starting it off is hit-and miss (a cart banging into an earlier cart still sitting on the shot ramp results in the latter cart getting pushed out of the gun on a diagonal), but once you got all three carts cycling, the gun _seems_ to be stable and yes, it shoots once every ~15 steps from a single barrel. Carts must be quite heavy - if a cart sitting in the load tile gets pushed, the standard momentum calculation takes place, and a water-containing cart obviously carries an extra 459 weight and won't feel much effect when pushed by a 28-weight oaken cart. Golden minecarts (empty weight over 750) work better.
You could still stick four of these together in the turret i've shown, for an output of roughly one shot per four game steps.
You definitely need three rollers to keep the triple-burst model going; water pickup randomly removes one or two depth units and thus the remaining friction and thus lingering time in the loading pond is variable; collisions will happen sooner or later, but i _think_ with the rollers, the carts are able to sort themselves out. To stop operation, a lever-operated retracting bridge on the shot ramp is needed. Take care in which order you remove the carts when stopped...