I've had several bolt action barrels cut from Green Mountain blanks, as have several friends locally. In our experience they are GTG.

A friend who cuts his own barrels received one blank with a bore that was tighter than spec (out of 10+ that he's purchased from them so far), but GM was quick to take care of him and easy to deal with.

I don't have any experience with their AR15 barrels, but I wouldn't hesitate to give one a try based on the above.