A reader sends this:
I think the Army will have a problem with terminal ballistics (wounding/killing potential) with the XM8.
The 12.5 and 9 inch barrel lengths are generally too short to provide the velocity needed for bullet fragmentation in the target at reasonable combat ranges. Your site viewers might be interested in the excellent discussion of the 5.56mm round (history, ballistic performance, gelatin testing, etc.), which is pinned as the reference FAQ in the Ammunition forum at AR15.com
I’ve just barely scratched the surface of that site. Tons and tons and tons of good stuff. You’d need an Energia super booster to put it in orbit, and even then you’d better hope they did the English-metric conversions right.
Go check it out. And a big time thanks to the reader who sent it in.
UPDATE: And I agree 100% (speaking as an armchair analyst) with the barrel length issues. The baseline carbine variant of the XM8, which would be the “standard” variant, has a barrel that’s two inches SHORTER than the current M4. Nicer to carry in a vehicle, I’m sure. But it’s supposed to be all the better to shoot you with, my dear.