Another idea of mine stolen---before I even had it

We proposed a new service carbine cartridge here, based on the 5.45x39.5mm cartridge case (rather, a 6mm PPC case) opened up to take a 6.5mm bullet with a high ballistic coefficient, yielding a finished cartridge that will still fit in the AR-15 magazine and action but deliver lethality far beyond the 5.56's practical range. Aside from the new barrel, the AR-15 would need to switch to the breechbolt compatible with the 7.62x39mm case, and take a retuned recoil spring and buffer.

Am still way behind on my reading: it was on the API list four months ago. Alexander Arms has rolled one out, along with an AR-15 upper to shoot it. They call it the .26 Grendel. They use a Lapua match bullet with a BC going North of .53. Check the video at this last URL, where an Alexander Arms spokesman describes its performance.

Here's a discussion of the 6.5 versus 6.8mm Remington SPC. Because the SPC appears to rely far less on barrel length, and SOF operators will prefer short-n-handy and reach for a different weapon if the range goes beyond 400 meters, 6.8mm will be the new US service rifle cartridge if the 5.56mm is to be replaced. But a .26 Grendel upper, which was once Fûz's pipedream, now has become a commercial off-the-shelf item.

The US Army consideration of replacing of the AR-15 is also proceeding apace, and apparently the teams are talking to each other, because a 6.8mm XM-8 is in testing (scroll to post dated 9 March 04).