It's $Pspew, not $particle spew or anything like that. As I understand it:
$Pspew:
+Count: 1 - number of particles emitted per cycle
+Time: 16 - time in milliseconds between cycles
+Vel: -0.45 - particle velocity as a factor of the host projectile's velocity, a negative value makes it go backwards 'out' of the projectile
+Radius: 2.1 - the radius of the particle
+Life: 0.8 - lifetime of particles in seconds
+Scale: 0.8 - haven't figured this one out yet
+Bitmap: particle_green - particle bitmap
You need to have 'particle spew' in the weapon flags for this to take effect.
EDIT I think the function could benefit from something similar to the $fof tag to specify a cone where the particles are emitted.