I use RRA on my BCM middy. I'm not crazy about them because they move quite a bit. Im used to a solid lockup like my Colt M4 and DPMS Glacief handguards. It's not a big deal but I would buy Cavarms Tapered handguards if I had to do it again.
On the other hand, my RRA handguards have little to no play on my BCM middy, whereas the plastic handguards on my 6920 have ALOT of play, as have most other 6920s I have handled.