so to resume fast : you can add particles to a surivor, zombie..etc except some props maybe , have to try, but l4d2 got a list of particles ( smoke for explosion ..etc) here is a link :

https://developer.valvesoftware.com/wiki/Talk:List_of_L4D2_Particles

Before making this weapon i had the idea of a electric projectile, so i had to attach a bone ( can explain more in detail later) to the projectile and i attach a particle to this bone, so when you shoot with grenade launcher you see the particle flying away.

I had to find if any particle from l4d2 works ( not always the case) and a good looking one, search electric, sparks ..etc

When i found the good one, only after this i started making the gun 3d model.

I don't think you can use this method for firing bullets because it's already a particle not a physical object, but you can edit particles and create custom ones, i'm not sure how but it"s totally possible.
You can find some star wars laser stuff ..etc on the workshop for exemple.

Thanks for the tip but sadly I have no expereince on making workshop mod for any games at all,I would love to more sci fi grenade launcher mods like this again. If you have time can you make a new grenade launcher mod like this one?

Trying to make original mods, so maybe not a grenade launcher again, but who knows?