There's a guy at AR15.com trying to get H&K to let some of their gas piston upper out for sale, no news yet, publicly they have said they won't sell to Civies, I supect that's just till the U.S. Military gives the final OK on buying they XM8.
Another guy is making a gas piston upper kit, it was posted here a few days ago, I'm not sure when those will be available.
I'm also not sure on the status of the 6.5 Grendel.