When conditions are right, bullets are easy to see in flight. The sun has to be right, and one has to stand in the right spot. At the range, if one sees a bullet once, then one can stand in the same spot and see bullet after bullet after bullet, until things change like a cloud comes by or the sun moves on etc.. Seen bullets can be super fast magnum bullets, or hand gun bullets or rimfire. Writing this made me realize, i haven't seen a bullet for quite a long long time.

I read one time in a magazine that one doesn't really see the bullet, rather, what one sees is the 'vapor trail'. Myself, i wouldn't know.