CMMG's newer spec'd barrels with the SPC II, 1:11 twist, 4-groove, chrome lined features came out late in the Spring and early Summer. They quietly moved to the preferred specs a little ahead of some of the other big AR companies like Bushmaster, DPMS, and Armalite. They do use 4150 barrel steel and MPI test their barrels. A few on the forum have purchased them recently, and we've seen one of their new barrels with CMMG's piston system upgrade.
Here's a post from not too long ago:
Their previous barrels were SPC II chamber and 1:10 twist. The specs on the newer ones are much better. 16-18 inch barrel with midlength gas would be good combos with their new specs.
There have been some discussion threads on AR15.com in CMMG's Industry Forum about these barrels. Plus, CMMG staff participate in their AR15.com forum if you wanted to ask them questions direct through AR15.com.