It's got a Red Dot I believe, without the addition of the old 3.5x like on the G36s.
It was primarily designed for reliability, and easy cleaning. It strips down real easy, in only a matter of minutes. I haven't seen the test results from sustained fire yet, but knowing HK, it'll be some impossibly large number. They are also all easily modifiable to other weapons; Only takes a matter of minutes, and can be done in the field.
The XM8 has several variants; Sharpshooter, Carbine, PDW, and SAW. The Carbine (Which will be th basic soldier weapon) has a 12.5 inch or so barrel. The Sharpshooter is simply the Carbine, but with a longer barrel, 20 inches. The PDW has the stock removed and replaced with a cap, and has the barrel reduced to 9 inches. The SAW has a heavy 20 inch barrel, and a bipod. The stock is adjustable.
It'll come with a some under barrel accessories; At the moment there is a 40mm Grenade launcher (XM320) and the LSS (A 12 gauge shotgun). It's going to weigh about 5.7 lbs without Ammo. At this point, I will stop cheating and just link you guys to these;
http://www.hk-usa.com/pages/military-le ... s/xm8.html
http://www.hk-usa.com/corporate/media/p ... 111703.pdf