I was just about to post a mention of Frank's book. I have a copy and it's got pretty much all the history.
I own two MK 760's. One made in Fruithurst, Alabama and the other made in Panama City, Florida. I've not yet added a California gun to my collection.
While they can be a bit less refined than the S&W guns they can be made to run. The biggest issue is with the magazines. I have around a dozen MK magazines and maybe two of them are reliable. On the other hand, Finnish Suomi magazines will fit and function with just a bit of sanding on the front pad. I was skeptical until I got my hands on some. I've never had my Fruithurst gun run so well.
The Panama City gun is a bit more finicky. It has one or two mags that it likes. But it runs really well with those two mags.
One interesting note, when I first got the Panama City gun I noticed that the recoil spring was really strong. This makes me wonder if it was replaced or my Fruithurst spring was replaced or just worn out. I did replace the extractor spring in the Fruithurst gun and that helped with reliability. I trimmed a M16 selector detent spring to a proper length and it works quite well.
I will say this about the MK 760's. It's not an ideal first machine gun. Many do not run reliably out of the box. They're ok if you are mechanically inclined and have the patience to tinker and tune. I know the first ones made were the Fruithurst guns. Many of those were made with original S&W parts that the two partners sourced from the company when they bought everything to manufacture the guns. Some consider these to be the best model. The Panama City guns are a little rougher. The bolt is nowhere near as smooth sliding in the receiver. The charging handle is loose in the bolt and has rubbed a groove on the outside of the receiver. The California guns are the ones with the worst reputation. The reason being that they were up against the May 19, 1986 deadline and were cranking them out as fast as possible. They were also converting the semi auto guns they had in stock so serial numbers will have different prefixes depending on what model it was born as. The early California guns had stamped sheet metal magazine wells but later guns had wells that were investment cast and those have embossed letters rather than stamped.
Anyway, that's my experience in a nutshell. They're not horrible guns but there's plenty better. If I were going to advise someone on what should be their first machine gun it would be a M11/9 with a Lage slow fire upper. An Uzi would probably be number two on the list. Everything else just gets more expensive or more complicated.