The CMMG/DPMS uppers are supposed to have a true 5.56NATO chamber. I cant say that I've personally measured the chamber to assure this, but I can say that thus far, the upper I have has run flawlessly, with the short 120 round session I had with it.
The upper I recieved had a chromed bolt carrier, which I was not expecting, but does feel very slick and hasn't seemed to be a problem.
The upper is T-marked, came with an M4 profile, chome lined 1in7" barrel, and seems to be New-Bushmaster-Black in color. It was very slightly off in color to my friend's prebanBM lower I tested function with.
My only gripe is that I am going to have to use my +.040 sight post with it because I like the ARMS #40, and it seems that wherever my #40 goes, so does my taller front site post.
Also printed a cute little 3/4" group at 50 yds with my EOtech using XM193 ammo.
Overall I am very happy so far.
Tex78
FORGOT TO ADD:
I swapped over my RASII to this upper and after a reinstall because I didn't think I needed to read the directions,
it went on without a hiccup, locked to it in the standard super-rigid fashion.
I like this setup, but to make mounting a scope easier for accuracy testing, I will might go with a system like the LaRue, Troy, or DD in a 12" setup down the road. That is unless you allready have an ACOG, in which the RASII has the no issues to worry about.