You have asked one of the hardest questions, both to answer and to build. I used to think the SR25 based M110 was the hardest rifle to clone or build a military correct rifle. But, when you cross to HK, you are in an alternative universe.
HK does not export much, and what they do, the German government restricts. The US does not allow much in terms of imports, and what they do, they restrict. HOW IS THAT FOR A START ?
No, add to the fact that HK cloners are extremely picky. Most military clone enthusiasts would be happy with an MR556 as a host, and adapt the look. Most HK cloners want to replace the BCG to an HK 416. The cost to make each incremental change is large. VERY LARGE, and very hard to find.
Now, add in Regulation 922(r), which basically tells you that a semi-automatic rifle has to be made of greater than 50% US made parts (I am paraphrasing).
Once you embark upon this journey, you will spend dozens of hours learning, then making connections, then finding that one special part, and most HK416/417 M110A and CAG rifle builders find a network of people with various small parts and take two years in building, and might end up hiring a barrel smith to make a barrel, and will have spent six figures and will have a rifle they will hand down to their grandchildren.