Essentially, the idea is as follows: you have this 3x3 module that you can extend to create a bridge. People can walk over it just fine, except that, once triggered, the trap will lock shut and fill with magma mist, and then smoke. Victims are vaporized; you will find their belongings on the bridge but their body will simply disappear. (I figured this out once I had to engrave the 5th slab, but it was well above my expectations! I hadn't realized yet that my testing was disintegrating passersby.)
This works by dropping heavy magma-safe objects, such as statues, from above into a hole in the bridge (which is made of grates, to let the mist spread). Below the bridge lies a pool of 7/7 magma, and a floor hatch upon which spent ammo collects. Once the floor hatch is opened, the spent ammo drops onto another grate below, while the magma keeps flowing through the grate and collects in a buffer room from which it can then be disposed of (for example, by pushing it out of the map). The hatches can then be closed and the spent ammo can be retrieved safely.
The setup isn't extremely compact or cheap, spanning 5 Z-levels and requiring various mechanisms and the use of quantum stockpiles, but the 100% ammo efficiency and its ability to dispose of bodies while simultaneously looting them is remarkable and a great time saver in the long run.
My current design consists of three such modules linked together. They are operated from a control room in which a minecart evenly loads ammunition between three chutes. When a lever is flipped, the chutes open, releasing dozens of statues onto the bridge below, while the trap closes shut. The best thing is, you don't even need access to the trap floor to retrieve ammo, so it can be used continuously by separating the chutes from the locking mechanism (although it doesn't seem necessary for now).
This is a link to my design, color coded for levers:
http://stucameron.ca/dwarfCanvas/#f4babHere you can see a turkey, a baby Llama and a peacock standing atop the bridge.
The operating room, chutes loaded with obsidian statues (and the ammo retrieval lever covered by my cursor).
The instant the statues are dropped, a magma cloud engulfs the poor critters, then they get smoked alive. When the trap is done working, no trace of them is left. Were they dwarves, you'd probably see a few
[ here and there, as well.
The magma is collected directly from the volcano, and regulated by an obsidian floodgate.
Statues are recovered from the special reclamation room.
Excess magma is disposed of
by using exploits by carving a fortification on the edge of the map.
All said and done, I have yet to test this trap on real enemies. I'm not sure if their armor and weapons will melt away, but if they don't then it's jackpot!