aac tirant 9s

Ok my AAC Tirant 9s is close to being approved I hope at 5 months on the 29th give or take. So here's my setback I'm gonna use this on my pistol however I'd like to mount this on a 300 BLK AR and shoot subsonics but there's no adapter. I talked to AAC they said try QRF and they said get the 300...