In Tourian, near the second save station, on the lower-left wall, there is a turret shooting green disks. Normally, it shoots with about a three- or four-second delay between each shot. However, if you stand right next to it and keep shooting its projectiles, it will also keep shooting about at the same rate as you. Coupled with the fact that a power-up is almost always left behind by each projectile you destroy, this is a way to get ammo very quickly and efficiently.
Submitted by: Jim