pgreen, a little rough history here (I'm sure an RKI will correct the details):
Carl Gustaf designed the Model 45 (yes, it was adopted by the Swedish Army in 1945), a.k.a. the Swedish K, a great subgun that can use both 36-round stick mags or 72-round Suomi drums.
U.S. Special Forces used them in the '50s and '60s, but when we became involved in the Vietnam conflict in the late '60s, Sweden cut off all arms sales to the U.S. military.
U.S. military then turned to S&W and asked them to produce something like the M45, and they pretty much came up with a clone of it, the S&W 76. (The M45 was not covered by any U.S. patents.) In reality, because the U.S. military already had a lot of Swedish Ks, most of the 7,000 transferable S&W 76's went to U.S. law enforcement.
Meanwhile, several companies (four, IIRC) decided to clone the now-popular S&W '76 to supply the civilian NFA market and thus created the MK 760, a clone of a clone.
Lest we forget, John Stemple also came up with yet another clone but in .45 caliber, the Stemple 76/45.
Neither the MK 760 nor the Stemple can use the drum mags, nor can most S&W 76s.
Each generation away from the original had some changes, both for function and for ease of manufacture. And each new generation was cheaper than the preceding one, so assembly quality on anything except a C&R original Carl Gustaf M45 is an iffy proposition.
The good news is that since manufacture of new MGs was banned in '86, most of the existing MGs have been fixed and tuned, so they are usually less troublesome today than when they were new.
All will either run fine, or can be made to do so. All are fun submachine guns, and like all registered NFA all are appreciating to one degree or another.
Still, both the workmanship and the value of each generation is less than the previous one.
HTH.