For a midlength the heaviest I'd run would be a H. You may want to check the rear face of the carrier to make sure there are no burs which would mark the face of the buffer. Marking up the finish on the face of the buffer is not necessarily an indication of a problem as a lot of things play a part in the looks of the buffer. I ran a H in my Sabre midlengths for a bit, but it seemed to make cycling a bit sluggish with M193 and M855 ammo. For 25.00 it isn't a huge investment.
