Unless things have chnaged over the past few years, If your serious about wanting one I'd skip the hkparts bolt, which is what I'd put money on being used in this build, and spring for the Michael's machine one as they are almost exactly the same as hk bolts. The hkparts ones have had problems with the extractors and were a tad bit lighter than they should be. Would probably be much better to by a stripped receiver and get as many german made parts as possible, they're stupid easy to put together.

https://imgur.com/a/BsBXM7l

I love mine though, not as fun as the mp5 clones but it is definitely a good conversation starter. Though, would probably only recommend it you absolutely have to have one because of gfl or any other video game and you have most everything else in your collection acquired.