Without a sketch of your layout, i can only offer somewhat vague advice:- if impulse ramps don't work, they're not constructed the right way. To accelerate, a ramp needs a track branch to a wall and another to a "floor". The "floor" connection is the important one, it determines the acceleration direction; direction and even number of "wall" connections doesn't matter, as long as each ramp tile has at least one. The wiki has a picture of a proper impulse accelerator - if the cart doesn't eject its load, it's not moving fast enough. This ties in with the previous issue. The minimum cart speed for shotgun effects is reportedly 70k, i.e. a cart dwarf-pushed down a three-z incline. An impulse rail of four ramps should be enough. More ramps give more speed and thus a more powerful shot.- fortifications and the like only influence what happens to your payload _after_ it has parted company with the minecart. Whether you get a shot or not only depends on speed. If you bump the cart into a completely walled-off chamber, the ammo will be spilt all over the floor. Fortification "muzzles" don't need much refinement: you can bang a speedy minecart directly into a fortification under closed ceiling and your magma blob or other munition will fly right through.The stationary minecart seen in so many designs is only needed if you send the shooting cart across a hole in the floor for easy recycling: in that case, the cart will "fly" and would pass through the fortification. But it won't fly through a minecart in its way (and that cart counts as on floor after being hit and thus can't pass the fortification, either). If you use a different recycling method and thus don't work with a hole in the track, you don't need the stationary cart.