The materials cost the same, but what I mean is it’s not possible to have multiple materials assigned to the same mesh with Particle Meshes - if you have a mesh with multiple material ID’s, it’ll just share the same material across all mat ID’s. I doubt anybodies ever going to need two materials for a bullet or shell or something though… or if they do they’re doing it wrong.
You can add collision to particles but it’s not advised since it’s expensive, it’s much more performant to use the primitive shapes for collision. You actually wouldn’t be able to access the particle collision events outside of the particle (in blueprint at least), so you wouldn’t be able to do a lot with it anyway.
And yeah, particles can stay around for a while. However it’s worth noting the particle collision has no ‘radius’ as such, it’s just a fixed point in space so objects will clip through the floor and that can break the effect if it’s for large objects. What you probably want to look at for blowing up walls and the like is destructible meshes. All this stuff can be found in Content Examples btw