Check again, they used MG 34's and 42's. Any Star Wars fan knows better.
The Hogue grip is a common AR accessory, the sniper stock too. It's basically an exercise in getting recognition for the caliber, which is the old .300 Whisper renamed, a .30 cal bullet in a necked out 5.56 case. It's considered a new hot item to run subsonic suppressed. As a supersonic load, it's been out since the '70s and didn't get any traction then. Nobody was hunting with AR's, and certainly not with a poodleshooter caliber. .308 battle rifles were preferred.
Now that certain 6mm variants are out, suddenly the .300 is all over the net in the lastest marketing blitz. Doesn't hurt to read up on the ballistics and decide for yourself.