Reality vs Games:

• The XM8 series existed only as prototypes. Even though test results were sometimes a lot better than competing products, the US Army did not want it. Heckler & Koch continued the program for some time but stopped it finally. This rifle is not produced anywhere (afaik).
• grenade launcher cannot be attached

Difference to XM8

The XM8C is the compact version of the XM8 assault rifle:

• shorter barrel (9" instead of 12")
• foldable stock
• grenade launcher cannot be attached

Some games list it as a submachine gun, while others list it as an assault rifle.