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Snabb was a Swedish company created to market a system for converting bolt action rifles into semiautomatic rifles. The system was patented in the US in 1938, making this one of the very last attempts at such a conversion. It appears that the company made a substantial number of overtures to many different nations in search of a customer, as Snabb conversions can be found built on a wide variety of rifles - several patterns of Mauser, the US M1917 Enfield, 1903 Springfield, and in this particular case a Dutch Mannlicher. Not surprisingly, no country was shortsighted enough to actually convert its rifles into Snabb semiautos - you will understand why when you see how the rifle works!
The basic system is a gas trap one, but complicated by the use of a two-part system of interlocking ratchet teeth so that the forward blast of gas from firing pulls the muzzle cap forward, but it is the rearward return of the muzzle cap which actually propels the bolt rearward. The bolt remains a two-lug rotating type, with the manual handle removed and a screw cam added to the rear to convert the rearward movement of the operating rod into a rotary movement to unlock the bolt.
The strange stock is necessitated by the extension of the receiver to fully enclose the bolt's travel (not necessary with a manually operated bolt action rifle). With that extension, the trigger remains too far forward for a conventional grip, and a pistol grip is required. Snabb probably used the thumbhole style because it is stronger than a standalone grip. On this particular rifle, the original follower in the magazine is missing, but the rifle appears to have remained fed by 5-round Mannlicher clips. In addition, the rifle has been rebarreled in .303 British as part of the conversion, for unknown reasons.
Finally, it is worth pointing out that there is no "Snabb" marking anywhere on conversions like this one. They are quite distinctive and easy to recognize, but very difficult to research without knowning the name in the first place.
Thanks to the Dutch National Military Museum for allowing me access to film this rifle! Check them out at: https://www.nmm.nl