Video hereThis is an extremely weaksauce version of the weapon, as each minecart is loaded with only 25% serrated discs and there are only five carts instead of the twelve it could fit in the magazine.
It's built pretty simply, just a minecart shotgun with a magazine where loaded carts are lined up and pushed forward by rollers until the hatch blocking them is opened and they can all fire. Naturally I filled the magazine with magma Then you just dig a hole beneath the wall they crash into and it's ready!
Stuff to know if you want to make one:
1 The rollers in the magazine can't be at highest speed or all of them will fire at once and jam each other. I used low speed in mine.
2 The wall the minecarts hit to fire MUST be a wall and not a fortification, a minecart hitting a fortification at high speed when there is a hole under it will fly through the fortification.
3 Having ramps that go down and then up again produce significantly more speed than they use so you could use this to make a flat shotgun.
4 The ammo spreads out quite a bit, having the target area along a hallway lengthways would be absolutely murderous. Firing it at a large room like this is pretty much the worst way of doing it. Then again each cart can hold 500 serrated discs and the target area is only 300. It's already absolutely murderous.
5 If you created a vampire loading corps and burrowed them in the reloading area, with other minecarts delivering ammo it would work pretty well I think, that was my initial plan but getting the thing working took more work than I expected and now I can't be bothered.
Stuff I learned about minecarts:
If you have a roller underneath a minecart when it hits a wall and shotguns it will change the trajectory of the ammunition depending on the speed of the roller.
If you have a minecart on a highest friction track stop and another cart hits it they will both stop, so if you assign a minecart to sit on the stop forever anyone standing on the stop will be safe. You can then have them grab the cart and put it on the stop just fine (If they don't get hit by the next minecart).
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Making your own,
The Auto-shotgun has three main parts; the barrel, the magazine and loading area. I recommend making the distance between each part as small as possible to keep it fast firing, though only the distance between the magazine and barrel is truly important
The magazine is an area where carts are stored after loading. The magazine requires a way of pushing the carts onwards when desired and a way to keep them in place to be triggered by a lever.The best magazine design I can think of is a trench with rollers inside and a hatch over the ramp leading to the barrel.The hatch is better than a door because a hatch can't have it's closing blocked by a cart and be stuck open. A trench is good because it allows you to flood the magazine with magma. You could choose not to use magma in your auto-shotgun but you would have a problem of dwarves trying to retrieve the cart from the magazine and you would need to find a way to keep them out.The rollers in the magazine should be slow enough that carts have some space between them or they will be jammed, I have found that rollers set to low speed work perfectly even allowing me to fire only a single cart by ordering the firing lever pulled twice.
The barrel is the length of track where the cart is accelerated to speed, in my design it was a spiral leading up three levels with rollers and then a ramp back down. I point out that rollers alone will not get your cart to the necessary speed, ramps are needed. My advice to try out for a barrel would be a track with a series of 2x1 dips with ramps inside as these give a cart quite a lot of speed, note I have not tested this design.
The loading area needs to allow you to collect the carts and start them again, if you want you can skip this part and just lead back to the magazine if you just want a magma machine gun. For safe collection of the carts you should have a length of track to hold carts that come from above that carts can then be taken and put back into place, the important thing is getting the cart out of the way before the next cart comes along.
A note on reloading, in my design I have cart tracks that bring the ammo directly to the tile next to the loading area and this works quite well, all my peasants just have to carry the ammo one tile and it works pretty good. The main problem is collecting the ammo after firing, I think that if you were to turn the target area into a stockpile and have that stockpile feed into the carts it would work quite well.