So I've been looking around the forums, and while I found a thread with plenty of speculation as to how a railgun would work, I have been unable to find a thread made after the introduction of mine carts that has truly applied dwarven science to the Dwarven Railgun.
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A quick forum search for "Minecart" leads to:
SCIENCE: Quantifying minecart physics
Perpetual Motion Minecart Grinder & Other Weird Physics
Minecarts are for what? No, really
The "How Does Minecart" Thread
60 pages of minecart science and testing, some of which is applicable specifically to railguns.
There is quite a bit of research that has been done, but by no stretch of the imagination is it complete. I'm thinking of something along the lines of the
Weapon Research and the
Science of Fall Damage threads. Personally, I'm probably going to conduct some tests on dwarves (mainly immigrants) once I have a self-sufficent fortress specifically designed for the task (with invasions off, of course). Stuff like the average damage taken from different types of minecarts, and testing EXACTLY how effective rollers are by themselves, and seeing the effects of different heights, as well as combinations with different types. For instance, is there a scenario where a light wooden (or even adamantium) minecart would be more effective than one made of silver? Obviously, these minecarts would have a much lower impact, but perhaps a speedier delivery might help with long-range accuracy, and if your plan is to douse a goblin siege with magma, speed might be more important than the actual impact, and it might have quicker reload (though wooden minecarts would be useless in the above example). After all, you're going to be able to retrieve the adamantium minecarts (assuming you don't somehow atom-smash it), so a variety might be useful. Also, tests on skipping need to be done. We should try to discover the skipping distance over water and magma with different minecarts. Are they different based on minecart density, and if so, how much? Is it possible to manipulate this so you can make a minecart skip over, say, a magma pool as many times as possible, over as long of a distance as possible? We should remember that, as tempting as it may be to simply pound goblins, elves, or even HFS with steel slugs, railguns have much more potential than just direct attacks. Also, it might be fun (or potentially Fun) to load in some caged sperm whales and see how that affects things. Also, can we manipulate minecart derailing to, say, launch a dwarf OVER dangerous terrain and onto another track? Then you could have a dwarf-only entrance/exit to your fort that could be weaponized simply by changing the designation from ride to push, and adding some weight. Goblins, without minecarts, or even the knowledge of how to use them would stupidly stand on the track, giving you an easy killzone. This might also be a good use of minecart variety, even though heavier minecarts might have quicker acceleration, a lighter one, should, in theory, be launched farther. We should also check to see if fall damage can be dampened by minecarts if they land on another track. This could lead to more one-way transportation. Even if some of this stuff doesn't result in practical information, imagine the potential for what you could do in adventure mode! A pulled lever activates a series of latches which work as a timer. Meanwhile, the adventurer climbs into the minecart, and is soon launched over a chasm filled with magma, all the while shooting at flying enemies. And don't forget the megaproject potential. I for one would love my dwarves to be comforted by cooling mist that, instead of coming from one of those all-too-common waterfalls, comes from an automated minecart-railgun which launches a minecart across a pool of water, then turns it around and sends it back across the water. The best part of this design is, not only is it efficient, but with a few floodgates and pumps, you could easily drain the water and replace it with magma to weaponize it during a siege, making it not only an awesome, dwarf-calming construction, but an amazing front-line defense if you've called your dwarves farther in, effective against any mortal creature!
TL;DR summary: Minecart science is a novel, and we're still in the first chapter.