Ahhh, thank you!
I did indeed misunderstand something fundamental. That actually sounds like a very useful quirk, allowing guns from 2 z-levels to concentrate on the same kill zone.
Surprisingly, everything is working as intended now, and I'm quite satisfied. A pressure plate triggers a near-instantaneous high-velocity shot, followed by ~2-3 low-velocity shots in 44-tick intervals. A lever on repeat keeps a similar pattern indefinitely. Operator fatique introduces irregularities, but no time is wasted - if it's not shooting, it's accelerating to maximum ramp speed. Rapid fire mode is strictly inferior to a variant without the closed acceleration loop... but the option is there.
Shutoff via door seems fragile in theory (trigger not synchronised to cycle, so it can get stuck), but it hasn't been an issue.
***
For something with a more regular output, I had some hopes for the following:
FFF
^╝═
~ ╚
╗ ╚
╔D╚
╗╔╝
╗╚
╗╚
╚╝
F Fortifications, solid tiles above
^ Pressure plate over SE track corner
═ Impulse ramps
~ Channel. Below: NS ramp, infinite water supply
D Door over SW corner, linked to plate
It fires a decently-fast shot every 111 ticks or so, and I like how it looks in action... but I'm not sold on the performance. Larger, fussier, less flexible, unresponsive (only natural shutoff seems a hatch on the firing tile, meaning it starts from a standstill), it generally seems to do less with more and excel at nothing.