owd,
I recall seeing the machining of this area on the RBL when I was at CSM last May. Neither of the two parts were touched by handfiles and they have a better fit than the gun shown, if I recall correctly.

While, I'm sure you're right about this area being difficult to fit by traditional hand work, proper CNC work should make this a piece of cake. Aren't they using CNC to finish fit parts? Or are they leaving metal fitting to the hand work?