So, I just got my new upper today and tried to put the BCG in it and the gas tub and gas key do not line up?  When I ordered the BCG, I was sent two by accident.  I tried the second and got the same results.  I pulled the BCG out of my RRA and it fit fine?  WTF?  
I am going to call the retailer(very well known) where I got the BCG  tomorrow, but any thoughts in the mean time?

The upper is SD 20" on a CMT with a Samson rail.

sounds like your gas tube is out of alignment.  

The barrel nut probably was giving the installer a hard time lining up properly.

As for why your RRA BCG works.........

+1 on the gas tube alignment. It should be straight down the upper, which you should be able to see when the bolt / carrier is removed. As the cam pin and gas key both ride in a slot in the upper, it is basically impossible for them to be out of line with the tube unless the tube is not straight.

I am switching the old BCG from the old RRA to the new upper because it fits.  I am putting the new BCG in the old RRA upper because it fits.  I will funtion test them next time I go to the range, but I still don't feel good about all parts not fitting both guns!