Hmm....so somebody's finally going to buck the current STANAG pattern? Good luck to them. I'm not sure I'd want to invest in something like that.....
Has Magpul even made a bigger mag yet? I personally think it will be a flop unless it has the capacity to hold several different calibers. As much as I dislike the STANAG mag and think a slightly longer one is the answer to finding something like a 6.5G type round for military/widespread adoption....sort of a do-it-all smaller caliber round....I don't think 6.8 SPC is it and I don't think this particular 6.8 specific mag is going to be a good idea unless it also is the SAME size as the next, all-encompassing military round....and that's not happening anytime soon, nor have I even heard of real plans by the US .gov/.mil to test for a new round for several years. Personally, I think 6.5G is the closest we've got right now. Since we currently have different mag designs (in SS) for 6.5G, 6.8, and 7.62X39....I doubt the new round and these other 3 (if it's not ONE of them) is going to have the same design and be "reliable" either....so again, there's no real advantage other than it's polymer.
Also, people are in love with their giant collection of current mags. The reason the SIG556 came out in the US was largely b/c of the ubiquity of the STANAG mag. Same with the release of the MSAR, XCR, SCAR-L, etc.
Further....the tapered case shape of the X39 round doesn't really lend itself to the non-constant curve of the AR magwell and I'm assuming LWRC/Magpul aren't planning to make a specific magwell just for that round so there's no real advantage there. Frankly, an AR/XCR/ACR/SCAR lower that takes rock in AK mags is a better solution for that particular (X39) round.
For 6.5G and 6.8 SPC II....this may make sense.....but these are both still very much boutique rounds and with the advent of .300 BLK.....you're seeing a lot of market share pulled from the first two.....even though it really offers nothing ballistically over 7.62X39 (unless shooting suppressed) and it's a helluva lot more expensive....and it fits the current STANAG mag (though I *thought* I'd read of issues with it in a current Pmag).
This is definitely a wait and see thing for me. I'm skeptical, but it may take off....it will have 'arrived' if the .mil or LEO adopt it on a broad scale (think .40 S&W for LEO)....but again, I doubt that's going to happen. I wouldn't want to be an early adopter on this in case it's a flop.