RE: Slug guns...Hear the bullet hit?
There is no way you heard the target impact from a centerfire at 100 yards let alone 40 yards. I don't know what you thought you heard, but it was not the bullet hitting the target. At those distances the bullet is still traveling 3 times to twice the speed of sound. The bullet is in the target before you are done hearing the muzzle blast of the gun.
I highly doubt you can hear the bullet moving down the barrel of a slug gun either.
Paul