O.k., that's this line of design taken to its limit:
I had developed the ACP 45 from the Bofors, so took the low loader input speed of the latter as legacy, but of course that speed was the result of wanting to build it without rollers. It wasn't said that the "low" roller speed i was using was actually the upper speed limit for taking up water in the pond. So i rigged up a bare-bones speed tester, and found that the cart already picked up water when coming from a "high" speed roller; only "highest" is too fast to reload.
Time in the pond seems to be eight or nine steps when going through without other carts interfering. So the final design looks like this:
##F ##F
.WW. .╔═.
.▼▲# .║╚#
#▲▲# #╗╚#
#▲▲# #╗╚#
#▲▲# #╗╚#
#▲▲# #╗╚#
.SE# .╚╝#
.#.. .#..
E,W,S - Rollers pushing in those directions
F - Fortification
In the north, it's a 2x1 high-speed E->W roller. It pulls the cart off the shooting tile and moves it into the pond. Both rollers in the south are highest speed and only used to ease the priming of the gun.
Carts get thrown into the gun from the south, and if they encounter other carts there on not-propulsion track corners, you get a newton's cradle situation with backwards fire. Standard design premise: rollers on corners should be in line with the "input" connection, _not_ the "output". The latter generally results in diagonal movement and can cause unholy messes. Switching off the northern roller should switch the gun off, although with four carts stacked against the fortification, three of them sitting on ramps, so only the last in line would be safe for dwarfs to pick up.
I just stuffed four carts into the thing and called it a day. Carts keep jostling each other, so output is a bit irregular. Usually, it puts out eleven shots per a hundred turns, but sometimes, carts apparently fail to pick up water; i got a dozen or so rounds of eleven shots per hundred steps, once each twelve and ten - and once six (!). Median shooting period is nine steps. Due to the roller on the shooting tile, all shots are fired at an angle.
Video:
http://mkv25.net/dfma/movie-2666-parabellumsubmachinewatergunP.S. Whoops, forgot to install an off switch. Good thing i built this thing in a part of the map that i don't mind getting flooded and being utterly unusable.