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From a standpoint of "what is hardest to make?" it would be the bolt, carrier, barrel extension and some of the various bits to complete the rifle, like a gas tube.
The actions are pretty easy to make, especially if one is willing to section them in half to mill them on a plain-jain 3-axis CNC mill as a pair of halves, then screw/glue them together. Magazines can be injection molded or made on a bending brake and with a TIG welder.
But the bolt/carrier group and the barrel extension.. those aren't as easy to make.
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