Hijak86 discovered some of the warts you get with a pistol-length AR configuration. It’s a good watch though- the photo at the end is tremendous.
I can confirm that the concussion that people on either side feel is worse than the experience behind the trigger group. Muzzle concussions are notorious for giving your range buddies headaches and this is a great example. A kit like that really needs a supressor to be comfortable for the user and bystanders. Also there is the issue of barrel length. 5.56 rounds tend to tumble easily out of shorter barrels. They don’t spin enough times to stabilize before they exit the barrel. These are all reasons that drove development of the 300 Blackout (BLK) over at AAC.
If you’re interested in a kit like this or want to weather the wait times and signoff requirements for an SBR you should look into the BLK- it solves most of the problems shorter 5.56 encounters and performs better in many cases. They also don’t breathe fire like a funnycar.