One more comment,
If your gas tube is not perfectly aligned from your gas block...back to the upper reciever, you may have problems lining things up due to the fact that the locking bar on a RAS straddles the gas tube by the barrel nut.
Another observation in reguards to the NATO STANDAG rails on the RAS and the upper reciever. Mine are perfectly in line. I'm not sure if it is because of the heavy barrel, but when I had the RAS mounted on a standard M4 Carbine barrel it was off by just a hair.
The way I tested it was to place my aimpoint on my upper reciever and look through my iron sights. The red dot was in perfect alingnment with the front sight post (both vertically and horizontally). Then I mounted the aimpoint on the RAS and looked through again...the dot was in the same spot, perfectly aligned with the front sight post (vertically and horizontally).
When the RAS was on the stanard M4 barrel, the dot would be off a hair (horizontally) when the aimpoint was mounted on it (as compared to the upper reciever).
Since it has been stated from other posts that the RAS will fit "pencil" and "standard" barrels...this implies a "range" of circumferences that the RAS will accept. This specifically pertains to the front portion of the RAS where it mates to the barrel. I submit that the Heavy barrels (Bushmaster) are at the upper limit. Since it mates perfectly to the heavy barrel, it may account for the rails lining up perfectly with my upper reciever.
...but then again, it could be because I built this rifle myself and the barrel/barrel nut was indexed and torqued correctly.