Speaking of magazines, this gun uses M3 "Grease Gun" magazines, and it is just one of several firearms which use these magazines. Aside: The M3 "Grease Gun" is a cheaply-built submachine gun created by the USA during World War II and used in military service for decades, and many thousands of GI M3 magazines exist, making it a natural choice.
I weighed two empty M3 magazines, and the average weight is 0.77 pounds. Magazine capacity is 30 rounds, and each mag is about ten inches long. Loaded with 30 rounds of 230-grain hardball (full metal jacket) ammo, the magazine weighs in at 2.16 pounds. So - once you cram a loaded mag into it, this gun weighs more than ten pounds.
Something I did not expect (though perhaps I should have) is that, when loaded (even with as few as five rounds), the magazines swell sideways at the top. This makes it difficult to insert the mag into (and remove it from) the gun. When empty, they don't bind at all.
These guns were made cheaply, without a whole lot of precision. In examining this one, I found a number of soft sheet-metal parts that had been battered a good bit - and all of them were part of the fire control system. I tempered these parts by heating their critical surfaces red hot with a torch and plunging them in oil. Only time will tell how much good that did.
Visible in the photo is the trigger-block safety, just above and aft of the trigger. Push it from this side to put the gun on safe. Forward of there, you can see the magazine latch, which moves outward when the button is pressed from the other side of the gun.