You can replace the material in the base projectile blueprint with whatever you’d like. I created a quick bullet in blender and imported it over to replace the ball. Just check the components of the blueprint and you’ll see where the static mesh is attached.
The thing that had me stumped for a quick minute was the fact that the scaling on the ball was at like, .1… so remember that when you import your own static mesh to replace it
Here’s a couple tutorials that helped me quite a bit as well: