A class II dealer didn't know that was a pretty common Colt "semi circle carrier"? It's made that way to completely defeat converting to full auto with a drop in device like a lighting link or something similar. It should cycle better/faster with a lighter carrier.
It "shouldn't" be giving you any issues because it was designed for your rifle..but stranger things have happened. There is a MASSIVE array of quality AUTO carriers out there from places like Rainier Arms and Bravo Company just to name two. But if you want a factory Colt Auto carrier, go to Specialized Armament. They have Colt FA carriers stripped with key for $100 (<EDIT>
just looked at them from SAW, price went up to $125
). Just pop in your bolt and guts from your other carrier and your GTG.
pretty unlikely, but there may an issue with your gas port as well (too small, severely carboned up, submerged in water and left to rust by previous owner.)