There are a NUMBER of ways to do this.
The image below is a still shot of a scene I did for the NAVY back in 2000 for a recruiting ad they were doing.
Here is how I did that 2 1/2 years ago with Maya:
1. I grouped a directional point emitter to the back of all of my bullets so that they would emit in the opposite direction of the bullet's movement.
2. I then made a "disk" object, shaded with my custom "translucent-glass" shader found here:
3. I then instanced the disk object to the particles, and assigned a <rand> (random) expression to the emitter so that it would emit the instanced "disks" at random x/y scale sizes.
I can't remember the expression, but the Maya docs, or an expression guru on these forums should be able to write one for you
4. I then rendered the particle "trails" separatley, and comped them in post to have more control over shiny-ness, blur, etc.
That was the short and skinny of it all
ALSO, for the sake of speed, here is another way:
-A QUICK, ONE dimentional way would be to use particleIllusion, (AKA, the default particle system in Combustion)
1. map animated frames of your rendered "trail-disks" to a sprite-emitter in particleIllusion.
2. track the bullets on your source footage, and using that tracking data, assign the emitter(s) to that data.
3. Tweak your particles to emit a "random" scale-emittion.
Meaning, adjust the settings to emit sprites at random sizes.
Hope that helped a little more
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS