Never fired a gun have you.  Note, that's not a question.

At 300 yards the bullet arrives a relatively long time (if you're the target) before the unsuppressed sound does.  Even when the sound arrives virtually nobody will know what it is.  Most of the reports from Las Vegas said that people thought the shooting was firecrackers until people started dropping.

A "suppressor" drops the sound of an AR-15 down by about 20dB.  To a level about 10dB higher than a jackhammer working concrete.  Movie "silencers" don't exist.  You'll hear a suppressed 556 or 762 at 300 yards just fine.