We were asked that yesterday and we pontifically pronounced, “it fired from the open bolt in automatic mode, and from the close bolt in semi.”
Then we rested back on our laurels as Gun Expert and —
“Well, how did they make it do that?”
“*!” Hmm… How did they? “Let me get back to you on that.”
Fortunately, several references on the shelves explain it in terms our walnut sized brain could grasp. It turns out it was very simple, when you consider how complex some of the other design options made the FG. And it imposed some trade-offs, costing the rifle significant semi-auto accuracy as the price of that mechanical simplicity. Let’s walk you through it.
It worked exactly the same on the First and Second model of the FG, by the way; so we will use images of both in this post.
The selector switch is on the left side of what we’d call the grip frame. (The German manuals call this part the Lager which can mean holder or receiver, too, but we’ll stick with “grip frame”). The selector swings through 180º of travel; knob forward covers an “E” for Einzelfeuer (“single fire,” semi-auto), and knob rear clicks on to “D” for Dauerfeuer, (“continuous fire,” automatic). Note that the letter that shows is the antonym of the function you get. Don’t ask us; Hermann Göring was not available to take complaints.
Comparing the Bedienungsanleitung (manual) image of a First Model to the photo of the second model above that, we can see how the trigger works. The trigger pivots on a pin forward of, and slightly below, the selector switch. The axis of the selector switch is also the axle of the sear (in the diagram, Part B8 Abzughebel, literally “trigger lever”). The sear nose (Fangnase, “catch nose,” B8a) is the hardened end of the sear that engages a notch (if you learned engineering English in Britain, a “bent”) in the operating rod (Verschlußführungsstück, “bolt guiding piece,” Part D10).
There are, however, two notches in the op-rod. One is towards the front end, and mostly right of center. One is towards the tail end, and mostly left of center. You can make out the two notches in this Forgotten Weapons photo.
Rotating the selector moves the sear laterally either right to align with the front-end notch, or left to align with the tail-end notch. If it aligns with the tail-end notch, a disconnector (Unterbrecher, literally “interrupter”, B9), works by disengaging the trigger from the sear until the trigger is released (i.e., normal semi-auto trigger reset). Thus the selector engages the sear nose with either the nose-end notch, which holds the op rod and bolt assembly to the rear, or the tail-end notch, which holds the op rod and firing pin only to the rear, allowing the bolt to lock fully into battery.
Releasing the trigger releases the op-rod, then. If the weapon is on full automatic, the bolt and op-rod come forward, the bolt locks, the op-rod finishes its full travel, and the firing pin initiates the cartridge. The whole thing cycles again and continues to do so until the operator releases the trigger. When he does, the bolt is held in automatic battery — to the rear.
If the weapon is on semi (selector knob swung 180º to the front), the trigger releases the op-rod, which brings the firing pin down on the primer. The bolt then cycles, but returns to semi-auto battery, closed bolt on a live cartridge, regardless of trigger position. The disconnector rides in the notch forward of the rear notch (here “bent”) only to disconnect when in Semi.
If you’re feeling envious of FG-42s, you can buy an excellent semi repro from SMG Guns, you can pay more than a new luxury car for a transferable, or you can take the following image, a pile of steel, wood and aluminum, and a set of files and try to do what SMG did:
It may take a while. Best of luck to you!
Now, the FG42 wasn’t the last word in open/closed bolt hybrid firing mechanisms. As mentioned, having the whole op rod and firing pin move was inimical to accuracy. This not only increased the motion of the firearm on firing, but it increased lock time substantially, giving that motion more time to work on sending your projectiles wild. But that was a tradeoff that designers at Rheinmettal accepted for their simple and reliable open/closed bolt mechanism.
As we’ve seen, waste heat is a real killer of combat weapons in automatic fire, and by extension, a potential killer of the men who fire them. Firing from an open bolt reduces the incremental temperature increase per automatic round fired, by allowing more air to circulate and more of the potential radiative area to be exposed to ambient-temperature cooling air. This has the side effect of moving the critical temperature area or point further up the barrel from its usual position 5 to 8 inches in front of the chamber.
Firing from an open bolt also prevents cook-offs. Contrary to common misconception, cook-offs are usually not instantaneous but result from a round remaining chambered in a hot barrel for some seconds or minutes. For a cook-off to be instantaneous (and risk an out-of-battery ignition) the temperature has to be extremely elevated. For a routine cook-off, which can take some time to happen, the biggest danger is that no one is expecting the weapon to fire, and people may be in an unsafe position forward of its muzzle at that point.
The FG42 was a remarkably good weapon, like many WWII German weapons. Not good enough for them to win the war, fortunately; it was the very devil to produce (ask Steve at SMG!) and was produced in the sort of numbers that would be a rounding error, or the scrappage involved in training some new line workers, in American, British or Russian production. The US produced, for example, about 40 times as many BARs as Germany produced FG42s; Russian production of the pan-fed DP28 LMG was easily double that. (German production wasn’t as dismal as you might think. They produced more rifles and carbines of all types than the USA did. But they did have a tendency to engineer something very good, and then fail to build it in numbers that would make a difference).