You will need the Sentry 7 (the 7 designates the .750 barrel).
As mentioned, mine is the clamp on. It is on a 20" barrel with rifle length gas system. I've found that with factory loaded Hornady 155 AMAX, I can set the adjustment on 8 clicks (I believe 15 is all the way open). It will actually cycle on 7 clicks but I opened it up one more just to make sure. It will also cycle South African and PORT surplus on setting 8. If I run a suppressor, I can turn it down to 6 clicks and still function reliably.
As for the accuracy gain, no one I've talked to can definitively say the cause but a couple of theories were that being overgassed caused the BCG to begin moving before the bullet is out of the barrel...not necessarily unlocking but moving rearward to begin the process of cycling. With less gas and pressure the BCG stays still for a longer period of time and thus not having extra movement to affect accuary. Another thought was that dropping the weight from the factory Armalite gasblock to the much lighter SLR changed the barrel harmonics to one more suitable for my gun and ammo. Both are interesting but my small brain doesn't really put a lot of faith in either explanation or the blind luck involved in gaining accuracy. All I know is that my rifle (which was already fairly accurate for a "factory" gun) went from a 1 - 1.5 minute gun to a .5 - 1 minute gun with the AMAX. Even the surplus dropped from 2-3 minutes to 1.5-2 minutes so for now the only thing that changed was the gasblock so that is what I'm giving credit to.