This is a Bushmaster AR-15 .22 Long Rifle Conversion.  The lower is a standard receiver with a low-density and low-recoil assembly.  The upper is converted to .22 LR and the barrel is changed from a 1/9 twist, to a 1/18 twist, thus doubling the spin rate of the bullet.  The implementation of a GEMTECH Outback II silencer on this gas-operated firearm, used in conjunction with Remington .22 LR Subsonic ammunition proves to be one of the most silent firearms on the planet.  In this video, you are hearing the “CLACK” of the bolt automatically cycling, and then the “CLICK” of the trigger resetting.  It is so quiet, that it does not register on our decibel reader.  The bullet’s impact on a target is louder than its detonation and discharge.  (You should also  know that we had to turn the microphone and audio levels all the way up just to register those sounds.  That is why you hear that strange whining and humming sound.)