7" seems a little short for an 5.56 (.223) SBR. I would stick with 10"-14.5". Of course my 10" set up is a 9mm so there's no gas block, tube or key. I have 2 set ups for the 9mm, a 5" TROS 3lug suppressed(this runs flawlessly). Then I have a 10" Spikes barrel and I have a few cycle problems before the rifle warms up.
Then I have a registered RR pistol lower in 5.56(.223) w/ a POF gas piston 11.5" upper that has never given me one problem and it will eat anything I feed it. I'm waiting on my BATFE approval to SBR.