The Lahti pistol was the brainchild of Aimo Johannes Lahti, a Finnish soldier turned arms inventor. While there is some dispute it’s hard to ignore the fact that Lahti’s pistol uses a very similar locking block to the earlier Bergmann-Bayard pistols. The overall styling appears to have been taken from the German Luger, providing good handling and natural point of aim for then-standard one handed shooting. His pistol was adopted in Finland as the L-35 and when Sweden’s contract for the Walther P-38 was interrupted by WWII, they adopted the Lahti as the m/40. These were produced by Husqvarna Vapenfabriks through the war and a little beyond. A little more about the history can be found at C&Rsenal, but let’s talk about its unusual action here.
The Lahti is a short recoil design with an inverted-U-shaped locking block set over the bolt and nested in the barrel extension. This locks the bolt to the barrel extension for a short distance during initial recoil. As both parts travel rearward lugs at the base of the locking block ride in channels in the frame. These direct the locking block upward, releasing the bolt to continue back further after the barrel extension is halted. This feature can be seen, somewhat simplified, at the right side of the included animation.
Lahti was concerned about the semi-automatic action functioning in the extreme cold weather of Scandinavia. His fear was that the initial force of recoil would be used up in moving a frozen barrel extension, with not enough remaining to drive the bolt back. So he included an accelerator on the left side. This semi-circular “foot” is attached to the barrel extension and is struck against the frame when the extension is halted. This slams the other end of the accelerator into a notch in the bolt face, “kicking” it with some of the leftover energy of the collision. We can see it working on the left side of the animation.
Finally, while we’re talking about the Lahti I should probably go on to say it is a pleasant gun to shoot. Its heavy weight paired with the 9x19mm cartridge, steep grip angle, and prominent sights make it stand out against other pistols from the era. If you ever have the opportunity please do try one out.