Troy 7.2, 7.6, KAC 8", any midlenggth forearm (9"), or a Daniel Defense MK18RISII which is 9.5"
I used a Daniel Defense MFR 12 and cut it back for one of my 10.5s
Two different barrels and QDs (FH and Brake) in these photos:
At one point I ran a TRX VTAC 9