It is said that better than average eyesight is a prerequisite to good shooting, though no doubt some get by with better than average luck. In any event, I know it is possible to see some projectiles in flight.
For instance, it is possible to see a 105-mm howitizer projectile in flight, provided you are standing behind the gun (the best place to be). It is also possible to see very easily a 4.2-in mortar projectile in flight from the side but the projectile is on moving probably at about 800 fps. A howitizer projectile is going maybe twice as fast on a good day. All it looks like is a black dot going away from you.
It has been a while since I've fired a .45 auto but I suspect it would be possible under the right conditions to see the bullet in flight.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.