The relief cut on the OP's barrel is interesting. I have two DD CHF barrels purchased about a year apart and neither of them has a relief cut at the shoulder.
http://i1363.photobucket.com/albums/...ps0d0fdda8.jpg
The gap, or lack thereof, is determined by the gas block manufacturer. Noveske/Vltor gas gas blocks are designed to be installed flush with the shoulder. Even if you offset a gas block that isn't designed to be by 0.030, given a plenum diameter of 0.162+-, you will still have about 0.020 before the barrel will occlude the plenum in the gas block.