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[Lozzymandias]

Trying to build a minecart shot!fun! in 43.xx. Before this design was adequate:

       XXXX
       +.^^^
       XXXX

. Open space
X Wall
^ North west impulse ramp
+ Fortification

To eject creatures forcefully to the west, drop the minecart down by a level, and return it to its start location.

There days instead of flying into the open space, derailing against the minecart, firing its contents through the fortification and dropping down, the minecart sails through the fortification as if it wasn't there. What gives? How can I deal with this problem? Any solid wall I were to introduce for the minecart to derail against would also cause its load to be fired into the wall surely?

[PatrikLundell]

I've never built one, but what about replacing the open space with a retractable bridge that was extended at the time of impact?
Another possibility might be to replace the hole with a NW (impulse?) ramp and a hole to the north of it?

[gchristopher]

I don't recall that particular design working before as a shotgun.

My reading of that design is that the cart traveling west would go airborne on the open space, and still be a projectile as it passes through the fortification. I think I've used that as an airlock before, where a dwarf riding the cart passes through the fortification without creating a path in from seriously dangerous embarks.

IIRC, The reason it doesn't work is that change from being a minecart (moving along tracks) to being a projectile (airborne). They're actually two totally different objects with different physics. The minecart stops being a minecart, and is briefly treated by the game as the same as an arrow or thrown object. When the it strikes something that stops a projectile, it turns back into a minecart and ejects its contents if going fast enough. Since fortifications don't block projectiles, the minecart will fly right through it the same as an arrow or bolt would. This is the same reason shotgun contents or creatures can be shot through fortifications. While in flight, they're actually in a special state of matter called "projectile." I don't know what state an object is in when it's skidding along the ground.

The cleanest design I can think of offhand that does what you want would be to move the fortification one space west, and put an empty minecart on the space currently occupied by the fortification. That will eject the shotgun contents without teleporting them up a z-level. The hole is optional in that setup. Larix showed you can get a higher firing rate by having the cart roll away sideways, and not paying that 11-tick delay for the cart to begin falling into the hole. I guess that only matters if your shotgun is also a machinegun or a squirt gun.

[Insert_Gnome_Here]

You can always drop the whole thing down 1z and replace the fortification with a wall.

[anewaname]

Read the Bugs section for fortifications at the bottom of this page. I know nothing about minecart weaponizing, but recall that fortifications allow some things to pass through.

[Fearless Son]

What anewaname said, most of the bugs apply to creatures that leap or are thrown.  I suppose that might apply to minecarts which are also airborne. 

The suggestion of a retracting bridge (or a floor hatch or a floor grate) at the bottom to keep the cart from flying off the rails (as opposed to just slamming into the fortification) would be the way to go here.  You could even sync it up to a trigger on the cart tracks so it automatically sends an open signal ahead of the cart.  The delay should give the cart time to impact before the trap door opens beneath it to drop the cart back down.

[gchristopher]

No, it's not a bug with fortifications. An airborne minecart is definitely something you want to act as a projectile, because you're often shooting it at people through fortifications.

The answer is either:

[fortification][stationary minecart][hole in the floor][track]

Or go look carefully at this post by Larix, where he demonstrates a minimal cadence single-cart repeating water gun, and has the cart colliding directly with the fortification, with a floor overhead to keep the ammunition from teleporting up 1 z-level. In that case, there's no hole. Then you use a track/ramp to move the cart after it has fired.

I think the retracting bridge idea would work, but will require careful timing and might be unnecessarily complicated.

I've spent a little time killing things with minecarts. In his above post, Larix showed that the max cadence is no faster than 5 ticks, but nobody has yet posted a max-velocity max-cadence multibarrel repeating gun that correctly uses checkpoint effects, etc, for any recent version of DF, so "the best" weapon design is still up for grabs.