So I rappelled about 40 times from 6' today with the oplux/ATC Sport. From what I've read and seen in videos, belay devices are supposed to give enough friction to hold you in place by just holding the tail end straight down, and the autoblock is just a backup to stop you if you let go of the tail. The Sport/oplux combo definitely did not give me enough friction to hold safely in place. I tried a 1/2 soft sheath rope and you could bounce in place and the ATC held great, so the diameter and smoothness of the oplux definitely has a negative impact on the ATC's functionality. Many posts in the climbing forums recommended using two carabiners with the belay device, to create more friction, like in the first pic. This definitely made it easier to hold in place when holding the tail end straight down, but still nowhere near enough to reliably hold yourself in place without the autoblock. Using the Sterling Hollow Block as the autoblock and two carabiners on the ATC, the line didn't slip at all and was a MUCH smoother rappel than the bumpy ride with only one carabiner. I'm also using my rappel rope as my tether and leaving the bridge connected to the Beal Jammy for the rappel down. I just held it loose in my left hand with my hand resting on the ATC and didn't have any issues with it catching. Since all my trees are bolted, even if it does catch, I can just step on a bolt to take the pressure off the prusik and loosen it. The two biners connected to the ATC aren't the ones I'm going to use, but were the largest ones I had so gave the most friction.
Has anyone here used the Edelrid Micro Jul? It's rated for 6.9mm to 8.9mm when using a single rope so might give that a shot.
www.edelrid.de