The M3/M3A1 fire from the open bolt. They have a fixed firing pin. The only way you can have a round in the chamber with the bolt closed is if the round fails to fire. I worked on Grease guns as a small arms repairman in the 70s in Europe. Rugged little beasts, generally. Slow rate of fire, compared to other SMGs. The only things that go wrong with them is bent stocks, and the tab of the spring that locks the barrel nut in place can break off.
The "safety" is the ejection port cover. There is a tab that sticks down, and holds the bolt in position (either open or closed) when the cover is closed. Rarely these do break, but only rarely. Any time you see a Grease Gun with the cover open and a mag in the well, it is dangerous. We have proven many times that a sharp jar to the gun with the bolt forward (cover open) can jar the bolt back far enough (without going back far enough to be caught by the sear) to strip a round from the mag and fire it when the bolt goes forward.
A simple design, constructed mostly of heavy stampings (made buy the Guide Lamp Division of GM, the people who made car headlights before the war), it is rugged, durable, accurate enough for close range, lighter and handier than the Tommygun. ALso a LOT cheaper. The govt reportedly paid a whopping $17.50 each when new. So I find the story of the depreciated one being $4.67 quite likely.
Grease gun mags are stout, and have heavily constructed feed lips, seldom giving any trouble (unless run over by a tank
), the bolt is massive, weighing nearly as much as the entire rest of the weapon, or seeming to. They are an effective close combat weapon, crude appearing, cheap to make, pretty durable (tanks excepted
) and a great value to the military for the money.
I don't know if any are still in front line service today, probably not. But 40 years ago, they were still the arm for armor crewmen, and I got to say, that if someone was trying to climb on/in my tank, I'd much rather be able to grab a grease gun than any M16 variant.