I do that in my fire class, spawnbeameffect is where the projectile (or in my case, instantfire extension) causes a hit effect to be spawned..Class'YourHitEffect' is the class that spawns the effect you want, I have a selection of emitters that spawn according to the surface type at HitLocation.
function SpawnBeamEffect(Vector Start, Rotator Dir, Vector HitLocation, Vector HitNormal, int ReflectNum)
Spawn(Class'YourHitEffect',,, HitLocation, Rotator(-HitNormal));
Hope that is of assistance I freely admit to being a total n00b with unrealscript and what I've achieved has been through the trial and error method.
If you want to spawn a decal there, take a look at the assault grenade and how that works (thats where I found it)