I had one once, as well. It's not a bad little buzzgun. Barrels can and do rotate during a burst, causing accuracy to be less than optimal. Many people have modified their guns to have an index pin to prevent this. Scott Andrey has done it a bunch of times. Inexpensive Swedish K magazines can be easily modified for use in the MK760. The Swedish mags are better quality as well. (Finnish Suomi stick mags are the same thing also, and are excellent quality) The rate of fire is a bit high for my taste (~800rpm), and it's difficult to reliably slow it down. The trigger is decent, and the selector switch usually works fine. I sometimes (rarely) got doubles when it was on 'semi'. It's a fun shooter, but I certainly wouldn't choose to use one in combat if better choices were available (STEn, Uzi, PPS43, etc).
Don't use hot ammo as you WILL cause breakage of the takedown pin and elongation of the hole. Someone made some buffers a while back. I don't remember who it was, but you can make your own from 1/4" neoprene sheet. They help a lot. As mentioned above, don't hold it by the magazine. Holding it by the barrel shroud is your best choice, until it gets too hot.
I can't tell you how many times I have heard people say "it's a copy of the Swedish K". It's not. Aside from the magazine and stock design, it has nothing in common with the Swedish K. None of the parts are similar, and certainly none are interchangeable! And firing them is a totally different experience!