SIG’s World War Two Semiauto Rifle: The Model U

The SIG company of Neuhausen Switzerland spent the 1920s, 30s, and 40s working on developmental semiauto rifles to sell both to the Swiss military and abroad. One of the experimental models in the succession of designs was the Model U, of which 16 were made in caliber 7.5x55mm Swiss. It was a gas-tappet operated action with a tilting bolt, and included a permanently mounted 1.8x optical sight on the left side of the receiver (the same type as used in the K31/42 marksman’s rifle). The Model U was made in 1942 and 1943, to typical Swiss levels of quality and precision. Like the designs both before and after, it was a valuable iterative step for SIG but not a rifle which would find any military or commercial sales.


  1. Looks promising but it needs a little more work. Does anyone want to try fitting a bolt cover to keep mud out? And how could the action be improved with respect to cost-effectiveness? I could recommend using stamped parts in low-stress areas…

  2. Tiny correction, Industrie was always spelled with an “I”, in many German fonts of the time, a capitol “I” just looks a lot a a “J” though, which confuses many anglophones. See the “8x57JS” misnomer for 8x57IS for another example.

  3. I’m not quite sure why they’d bother with the tilting block, other than it being not unlike the BAR lockup.

    Their “straight-pull” K31 rifle action, essentially a turnbolt with an extra campath to provide turning motion when you pulled on the operating handle, was already halfway to a practical gas-operated rifle, needing only a gas port and piston to move the operating rod backward and a spring to push it back into battery.

    All of which could have been produced on their existing tooling, unlike this prototype.



    • The K31 action would require several modifications before it could become the basis of a self-repeating system. First of all, the trigger group would need to move away from the bolt so that the user’s hand and eye would not get mangled whenever the bolt cycled on its own. Next, one would have to modify the receiver to cover the bolt so that mud doesn’t get in (AK or AR-15 styled receivers?). And then we have the elephant in the room: how to set up the gas system. Putting the gas cylinder below the barrel is intuitive, but then the operating rod would have to go around the magazine. Putting the cylinder on the right or left side of the barrel will have some balancing issues. Putting the gas cylinder on top of the barrel would be the best for having the operating rod push on the bolt, but would the Swiss want to move the iron sights far above the barrel? And do we have to mention the complications of annular gas systems? With all of that considered, what would be the best approach? Should we abandon the traditional rifle look and just make a new semiautomatic design that shares only the barrel, bolt, and magazine with the original K31?

  4. I am starting to worry about Ian
    Its scary when he says a rifle like this isn’t complicated
    Or is it me just getting old

    • The operating principles aren’t complicated but the assembly of parts in this case require expert craftsmanship!

  5. k31 semi auto: good point eon. i think you mean to use the k31 as a starting point, and not as a swiss howell smle conversion. what strikes me as odd, is that in 42/43 the swiss must have gotten all the semi auto rifles used by the different combatants, and still they come with this expensive model. i know they were not involved in ww2 and could afford a quality item like this, but still.

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