The Colt would be as is. The BCM would be identical to the colt except I think I am starting to like the mid, and that set up comes in 115 dollars cheaper than the colt (even cheaper if I go with/out the carry handle, but I am not sure what to get in its place). I am looking to have a carbine that can take abuse, and I dont think I'll be throwing optics on it any time soon. I saw the chart, and thats why I am looking at BCM, but to anyone that has both.. Is it better to buy the colt, or save the money and get the BCM? (not going to break the bank for another 115 bucks) Oh and are all non-HK ar parts interchangeable?