Depends on the shape and how the projectiles is traveling in a path.

The projectiles in my 2d space shooter game do well using just basic rectangles.

But as you delve into weird projectiles going a more complicated trajectory, throwing a spear in the air will need a different approach.

Bullets are small objects and are imperceptible by the human eye at normal speeds. There is no need to have a complex object to represent a bullet unless you are doing a "slo-mo glam shot". I would use a simple box for tracer rounds and nothing for normal rounds (rendering purposes only). The "tracer" box should be a specific size in two of the axis' and scaled in the other to represent the distance traveled between frames. For collisions, I would simply use line segment collision frame-to-frame.