If is indeed possible to see an artillery projectile going downrange if you are standing behind the gun but I've never been able to do that with a .50 caliber, never having fired one. It isn't difficult to see a mortar round either, even from one side. Not sure about any of these things coming at you.
There are even heavier machine gun rounds in use but I'd say the .50 caliber is pretty much the standard. Oddly enough, some countries made little use of heavy machine guns like that in WWII, arming airplanes with rifle caliber machine guns.
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.