How Did the FG-42 Selector Work?

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.”

This one's an SMG Guns semi clone. Pretty, though, innit?

This one’s an SMG Guns semi clone. Pretty, though, innit? Images do embiggen with a click.

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.

FG42-0034- grip FW

This image is from a crudely DEWATted Second Model FG that was examined by Forgotten Weapons. There’s a great set of images there, and the gun’s internals are mostly present and correct.

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.

FG-42 exploded view

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.

FG42-0003_FWRotating 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.

These schematics are from Allson & Toomey's Small Arms, pp. 226-227.

These schematics are from Allsop & Toomey’s Small Arms, pp. 226-227. The depiction of the selector in these drawings is how we came to understand that the selector (“change lever” in British English) covers the appropriate letter for type of fire selected.

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:

FG-42 Type II exploded view

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).

10 thoughts on “How Did the FG-42 Selector Work?

  1. Raoul Duke

    Hands-down, the best simple explanation I’ve seen of this mechanism.

    It’s also easy to see the future M-60 operating system in that diagram.

    1. Hognose Post author

      Even the spring that holds the M60 grip housing on is copied. But check out a Lewis gun sometime. Same op rod, same bolt….

      1. Kirk

        I’ve mentioned it before, but the mindless copying of the FG42 in the M60 goes so far as to include the firing pin spring which is there in the FG42 solely to boost firing pin energy in the semi-auto mode. Which added in an additional bit of complication and confusion that didn’t need to be there on the full-time full-auto M60.

        The FG42 is a lovely bit of design work. The M60? Not so much. The FG42 could be termed an improved realization of the Lewis gun, created by geniuses. The M60? The Lewis gun, as interpreted by a hydrocephalic chimp.

  2. Brad

    This brings to mind few questions.

    The first is, if cook-off danger is primarily from a round left set to cook for a long time, couldn’t using the bolt hold open on a closed-bolt automatic weapon be a practical means of avoiding that?

    The second question is, would elevating the muzzle of an open bolt weapon between firings, increase barrel cooling by means of chimney drafting air through the barrel?

    The third question is, how did the selective fire mechanism of the Japanese Type 64 rifle work? I remember reading somewhere the Type 64 also fires from the closed bolt on semi-auto and fires from the open bolt on full-auto.

    1. Hognose Post author

      1. It would help but there isn’t any weapon where the manual of arms contemplates this as something done regularly. On most closed-bolt weapons, you’d have to interrupt the feed, or eject a live round, to lock the bolt back.

      2. To some extent, sure. Anything that would increase airflow would help. On the ground Lewis, there were aluminum fins and a cylindrical tube and the idea was it drew air by convection in at he breech end and moved it towards the muzzle. I’ll see if I can find a diagram. But the problem is that the temps get so high so fast… the gun often gets unreliable before it fails outright. I’ve often wondered if heating from attempted continuous fire is one of the reasons the WWI aviators had so many jams (sure, crummy ammo was a factor, too).

      3. I dunno how that one works. Never actually seen one; think they seldom left Japan.

      1. Brad

        Thanks for the answers.

        As far as question 1, I was thinking that when replacing an empty mag on a hot weapon that the chamber be left empty until the need to fire was urgent. I suppose that would be more practical with a smaller 20 round magazine than the more typical 30 rounder.

        1. Hognose Post author

          It’s not like i’m left poor or anything, but a nice flow of six figures worth of toy money per annum was cut off, which gave me cold feet about a $5k Title 1 firearm. I have a new project which bids fair to generate some more toy money this year.

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