This is an updated version of a similar weapon on my gallery designated the M3. I never really liked the way the old design looked.
I could spend as much time describing the weapon as I did designing it, but I'll just go ahead and answer the main questions I suspect I will be getting:
Yes, 5.73x35mm is small for conventional ammunition; however, it should be more than adequate for a light assault weapon, considering that the HK G11 caseless rifle fires a 4.73x33mm round and has ballistics comparable to the M16. In 30 or 40 years, I'm sure that munitions technology will allow for smaller unconventional rounds that surpass their larger conventional counterparts.
The bolt shield on the rear of the receiver does not operate during normal firing. It opens only when the charging handle is pulled back in order to unload the weapon, eject a misfired cartridge, load the first round of a fresh magazine, or to view the chamber for inspection and cleaning.
Also, the two main reticles (i.e., not the backup red dot) shown for the ACOG are for two different models of that particular optic. The M986 CICOG, however, has selectable holographic reticles based around the black milling line etched into the glass.