Pistol not an SBR to keep it carry-permit legal; no brace until we get the ATF Jackboot BS with their impending attempt at a brace-ban sorted out. Once that's done I MIGHT consider an old-school SBM4, but that's more so I can stick a pouch with an onboard spare mag in the tunnel than as an extra point-of-contact.
Right now (other than WA's new Mag Ban keeping me from getting four EPM's for it) standard combat load for a GAU-5 is 4x30 or 120 rounds; with the added tenner in the well the PIP model goes up to 130, and a brace-tunnel 20 for a ready reload takes it to 150 rounds in the bag ready to rumble. Next concern is, any potential problems from leaving a BCG locked in Hold-Open for extended periods? (You have to lock the bolt open to rack a QRB-equipped barrel on, and the entire goal of this concept is to shave every possible second out from between "unzip" and "bang.")