Setting things up like this:
- a projectile set up to use a model
- use the PITCHFROMMOMENTUM flag in MODELDEF
- spawn the projectile (using SpawnProjectile in ACS)
- set the parameters so that the projectile is moving straight down - i.e. a speed of 0 and a negative vertical speed
Giving it a speed of 1 makes it point downwards.
Map attached. The first 3 switches (from left to right) will spawn a downwards projectile (v speed -64). The last 2 spawn a projectile upwards (v speed 64). The scripts responsible print the forward speed values. The left-most switch exhibits the kind of behaviour that I'm talking about.