Warfare for Writers - 4 is the latest update from the Timons Esaias' workshop. These comments go from medieval to modern times.
You can see arrows and crossbow bolts flying through the air, likewise trebuchet loads. And you can figure out pretty much where they were fired from, as a result. Also, you can hear them. Likewise with musket balls and cannonballs and artillery shells. You can see them, and you can hear them, and because they are subsonic you can hear the shot before the projectile gets to you. If it's close enough you can also probably see the muzzle flash of the gun, not to mention the smoke, and tell where it came from.
This all changed with the modern rifle, though. Modern bullets travel quite a distance at supersonic speed, so you actually hear the sonic boom of the bullet before you hear the shot, if you hear the shot at all. Oh, and the bullet will hit you, or go past you, before you hear it. And you generally can't see the bullet, ever. Because the sonic boom is the main sound you hear, and the bullet is past you before you hear it, the sound cues (unless you're VERY used to this) tell you the opposite of the truth. You will think the bullet came from where it actually went, or 180-degrees from the correct solution. When people hear modern gunshots, they almost always start looking in the wrong direction.
The reason we can be pretty sure there were no shots from the "grassy knoll," is because so many untrained people heard shots coming from the grassy knoll.
So unless your character is very close to the weapon when it discharges, don't have them figure out where the shots are coming from by the sound.