I bought a couple hundred rounds yesterday. I have a 6.8 that is supposed to be ready to shoot on Wednesday.
Regarding why Hornaday didn't offer a heavier GMX, I suspect it is because the all copper or alloy bullets are so long. I don't have any experience with loading the 6.8 but in some other, especially older, limited capacity cases mono metal bullets in the "standard" weights are just too long and eat up powder space and may be too long for the standard twist to stabilize as well. In addition, all evidence seems to confirm that mono bullets one or two weights lower than a cup and core, even a bonded cup and core, will perform well.
My personal experience is with the 458wm, and a 450gr mono metal will perform differently but as well as, maybe better than a 500gr lead core, with the 243 where an 85gr mono will perform at least as well as a 100gr lead core, and with the 308 where a 130gr mono will perform at least as well as a 150gr lead core.
FWIW, I think the typical mono metals are "harder" [in the sense of more limited expansion] than almost all lead cores, even bonded and partitioned A Frames, and rely on wound volume achieved via penetration more than a lead core, which relies on wound volume achieved by a wider though shorter permanent wound cavity.
Also FWIW, I choose bullet type based on game (duh!) but also based on the cartridge being used on that species of game. For example, If I'm using a 30-06 for whitetail deer, I am happy with Sierra GK's in 165gr [a boat tail bullet, which all but ensures cup and core separation when heavy bone is hit,] which is a "standard" weight for the cartridge, but if I'm using a 243, I like the Barnes 85grs, which is relatively light for the cartridge, but nevertheless delivers more than adequate penetration. As a complete aside, I do think Nosler got it about right all around way, way back when he made the first Partitions, which, imo, continue to almost perfectly bridge the gap between a good wide wound channel and deep penetration.
Edit to Add: History is chock full of cartridges with phenomenally successful histories on game which launched in their heyday and still launch cup and core bullets between ~2200fps and ~2700fps. My review of the 6.8 ballistics leads me to the conclusion that we are back to the future with the 6.8, but in the AR platform, rather than a falling block, break open or bolt action. Also, imo, the higher the velocity of the round the "better" the bullet needs to be.
JPK
