Almost Everything about the gun (inclusive if the pattern of the parts) is/are pretty much proprietary (which has it's drawbacks when there are no options except for the high dollar HK upgrades, and some geissele). Take a closer look at the gun, it's not standard mil spec pattern.
The bore Is uncoated and tapered. I'm sure the colt is chrome lined and non-tapered. The HK has an ambi safety and has a firing pin safety. The upper/rail heights are proprietary/different than all other ar15's. It's wierd, there are only 3 rails that will fit the gun (hk, geissele, remington defense). The lower May remind you of a standard mil spec lower... But it's not, it's thicker, has a more flared magwell, and some other aesthetic differences.
The other main difference would be the piston system. Obviously. And, the gas block has a release port.
I'm sure I'm missing other stuff too. When I got the gun I took it apart and was amazed at how much different their parts were. Can't run another barrel, rail, only some triggers work, etc...
Trust me kids, it's different than a mil spec pattern/dimension gun. I'm not claiming that the differences make it better (well, maybe some if them do...). I'm claiming that it is different. I'm also claiming that it is reliable. I've seen several colts fail to eject, mine has not. I'm sure there is an MR556 out there that has, I wouldn't be so ignorant as to say one hasn't.
You also have to realize that Frank is a ball buster. He knows the HK mr556 is superior to a colt 6920.... People like to hate on HK. It doesn't bother me.
Frank (I'm sure, jokingly) claimed the only difference was the roll mark. You asked me to point out the differences. I did.