Like user "mostpeople", I went through the same thought process when rewelding an MG42 and an MP44. I asked, but all I received here was fluff, or the advice to check on Weaponsguild or Weaponeer. Nobody would tell me explicitly what needs to be done to build a legal semi-auto from a parts kit. Who knows, maybe I wasn't a full fledged member of the local builders elite or didn't know the secret hand shake. I signed up on Weaponsguild and immediately regretted it. More fluff, and slow to load. Builders getting lost in minute details instead of providing a road map. Weaponeer wasn't quite that bad, but also slow to load. Luckily I found out that BRP had an ATF approved MG42 semi-auto conversion, was selling the conversion parts needed and also offered downloadable blueprints, including their ATF letter. As good as it gets.
The blueprints were mostly useless if you used other receiver flats than BRP's. As an example, measurements for holes and cutouts were relative to the front or rear of their receiver halves, not relative to the camming piece cutouts. My receiver flats were longer on both ends, so none of the measurements were of any use. There were measurements relative to top and bottom of the receiver. With BRP's receivers known to be undersized, these measurements were also useless for other receivers, let alone the fact that measurements were relative to rounded instead of plane surfaces, kicking the door wide open for all the mistakes you can make using a curved surface as a reference. Overall one big headache.
When I was almost ready to give up and unload this entire hairball on Gunporker, I sat down one more time and mulled over the operational concept of an MG42 and how parts have to interact for it to work. I started with the center line (because barrel, bolt and rails need to be lined up to work properly), measured every component and marked up the BRP blueprints with new dimensions relative to the center line. A butt stock locking piece with rivets and a bit of torch cut receiver attached to it helped me determine how much I had to cut off the rear end of my flats. From then on work progressed steadily. I also found out why some builders needed to extend the charging handle bar to keep it from popping out of the receiver. It was another byproduct of wrong drawing dimensions. The original MG42 bar never popped out, so why should a semi-auto bar pop out? These builders had cut the slot too long and too wide, almost 1/4" off. I milled as little as necessary and then hand filed for final fitting. The less I relied on other builders' dimensions and the more I used common sense, the better all parts fit together. In addition to BRP's design, I added a couple more obstacles to prevent the semi-auto from being converted to F/A without investing a frustrating amount of work.
The MP44 was a little more straight forward as I had a PTR44 as a template. The differences have been discussed here sufficiently so that I don't need to regurgitate. The only extra thing I recommend is instead of welding the auto sear slot shut, weld a blocking piece of hardened steel into the auto sear slot so it interferes with the auto sear hook on the charging piece. Then mill the auto sear hook off the charging piece so it clears the blocking piece. This way there's no easy conversion consisting of using an original WW2 fire control group and charging piece. It requires partial destruction of the rear receiver to get the hardened blocking piece out. For obvious reasons, the U-shaped denial inserts found in the back of the PTR receiver cannot be used in the MP44 build. That's why the blocking piece in the auto sear slot is so critical. Think about it. If you don't see the reason, put an MP44 bolt and charging piece next to their PTR44 counterparts and it will be very obvious