That bolt-blocker gate-safety wasn't worth it's name, was it?True enough, the M3 wasn't inherently safe. The little gun fired from an open bolt, which means that you pulled the bolt back and the bolt was held by the trigger. When you pulled the trigger, the bolt slid forward, stripped a cartridge from the magazine, and the fixed firing pin fired the round. As long as you held the trigger down, the bolt would slide back and forth in the tube-receiver and shoot bullets down the barrel.
The safety was the dust cover, which you closed. There was a little finger inside the dust cover that was notorious for letting the bolt slide forward when jarred. That bolt flew forward, the bolt stripped a cartridge, and fired it down the barrel.
The M3 was not safe. It was a submachine gun. Designed to kill people. Nothing safe at all about it. For that matter, the tanks we drove and used were also not safe. They were war machines designed to kill people, and they'd kill the crew just as mindlessly as they'd kill the enemy. The M3, and the M60 series tanks taught me that the only safety I could depend on was the meaty computer between my ears. To this day, I don't trust a safety on a firearm, and I damn sure don't trust a tank. They'll kill you with supreme indifference.
That being said, if you knew what you were doing, both the M3 and the M60 would bring you home, let you see your kids and wife, and tell stories where old soldiers gather. They were not safe, and the guns I carry today have fewer safeties than the weapons I used in the mid '70s. The Smith and Wesson M&P I carry every day has no safety, simply the knowledge that if I don't pull the trigger, it won't shoot a cartridge down the barrel. Guns aren't safe. The people who use them can be safe, but guns aren't. Don't be confused by a mechanical device.
As Mostly Cajun pointed out. "Is gun, is NOT safe." That's a good lesson for all of us to learn.