I've had a go at a 'MIRV minecart'. That is, a lead minecart containing three lead minecarts, each of them filled with galena. I let this one descend 24 ramps and then collide with a series of fortification rings. Different carts did indeed release their payload of ore at different layers of the fortifications, with some randomness over several attempts.
Tested and verified "Cartception"/"Yo Dawg"/Cart-in-cart-in-cart-in-cart-... in the arena.
Carts are discharged from parent carts like items, but then immediately begin acting like carts again. If three carts come out of one parent cart, they
tend to collide with eachother and only one shoots forwards.
If you nest single carts "Matryoshka doll" style, then each time the cart hits an obstacle, it launches the child cart (containing the remainder of the stack) onto the next z-level above. I was able to shoot carts over the pyramid in the arena this way. If the cartstack hits the side of a multiple z-level building at sufficient speed, it will unstack at a rate of 1 z-level per tick until it runs out of carts or the surplus carts can shoot over the building. If the cartstack hits the edge of the map, it will completely unstack and climb as high as it possibly can.
Further testing revealed that this happens with other tool containers (can't easily test non-tool containers in the arena). A large pot was filled with daggers by hand and placed into a minecart. The minecart was then launched at a 2z high wall. The minecart hit the wall at ground level and discharged the pot on the level above. The pot hit the wall on its z-level, and discharged the daggers over the wall. The daggers shotgun-flew out and got a kill on a carefully-placed goblin.
We have yet to test caged animal projectiles. I wonder if they would be released and shot out? Will we get a heavy-cage-full-of-elephants projectile, or a kitten shotgun? If that worked, would we call it "live ammunition"?