while i think what you’re suggesting is possible (using a particle mesh with collisions turned on to spawn a decal) i would strongly suggest doing it in a different way:
either, 1 - do a line trace when the gun is fired and spawn the decal at the impact location, then just play a muzzle flash effect at the gun muzzle.
or, 2 - spawn a projectile actor at the gun location and then spawn a decal at the impact point when the projectile hits a surface.
generally i’d use 1 for bullets from guns (ie things that move very fast) and 2 for missles etc. (ie things that move slowly enough to see.) There should be plenty of tutorials on youtube if you search for “line trace weapons” or “projectile weapons” in unreal 4.
Hope that helps!