Looking for a new upper assembly for a colt a2, curious about what would or wouldn't be compatible. I tried putting the upper from my rra lar15 flat top assembly onto the colt lower. the only difference is the take down pins the colt has a pin in the back like my rra. the front has a pin with a bolt inside of it though. Would there be a certain type of upper I would have to buy? Thank you for your patience to my ignorance.