I have built a generic bullet using a default sphere surrounded by a collision box. It is fired using ProjectileMovement.
However it does not collide correctly UNLESS the collision box is dragged over the default scene root. Then, it collides fine with the environment, simulates physics, etc.
BUT…
once the collision box is dragged up to become the default scene root, it loses all its scale and transforms and reverts to its original size, which then cannot be edited. It also drags the bullet with it, which then expands to fill the entire collision box. You can try to plug new scale/transform into the collision box manually, and while the numbers are changeable, they do not actually change anything.
I posted a video of this at