Compared to the previous Schmidt–Rubin series Model 1911 rifle and carbine the Karabiner Model 1931 bolt and receiver were significantly shortened, allowing for a rifle length barrel and sight radius, without increasing the overall length of the Model 1911 d carbine, moving the rear sight element closer to the eye, and cutting in half the amount of time for the firing pin to strike the cartridge after the trigger was pulled.
The Karabiner Model 1931 barrel is free-floating and has 4 grooves and a 270 mm (10.63 in) rifling twist rate. The action itself is only connected to the stock by two screws, one attaching to the chamber, with the second attaching to the tang. This allowed the Swiss to eliminate the aluminum barrel mounting collar used in the Schmidt-Rubin series. The trigger was also redesigned.
Karabiner Model 1931s are noted for their excellent accuracy for a service rifle and quality. The Swiss armed forces considered individual marksmanship to be of utmost importance. Therefore, the K31 was made with tight tolerances and excellent overall craftsmanship. According to the Swiss Military manual for the Karabiner Model 1931 using standard issue 7.5×55mm Swiss GP 11 ball ammunition in a fixed mounting the expected accuracy of fire at a range of 300 m (328 yd) is 4 cm (2 in) (R50) in the horizontal (windage) axis and 6 cm (2.4 in) (R50) in the vertical (elevation) axis. Accuracy of fire at a range of 1,000 m (1,094 yd) is 21 cm (8 in) (R50) in the horizontal axis and 43 cm (16.9 in) (R50) in the vertical axis. R50 at a range means the closest 50% of the shot group will all be within a circle of the mentioned diameter at that distance. For reference a 1 minute of arc (MOA) circle at 300 m (328 yd) has a diameter of 8.72 cm (3.4 in) and at 1,000 m (1,094 yd) has a diameter of 29.08 cm (11.4 in).
K31 straight-pull action system
The Karabiner Model 1931 is noted for its straight-pull bolt action, meaning that the bolt handle is pulled directly rearward to unlock the action and eject the spent cartridge case in one motion, and then pushing the bolt handle forward again to chamber a new cartridge, cock the striker, and lock the action, rather than being manually turned and pulled back and forth, as in contemporary bolt action service rifles, like the German Karabiner 98k, or the British Lee–Enfield Rifle No. 4. A straight-pull bolt action reduces the range of motion by the shooter, with the goal of increasing the rifle's rate of fire.
Unlike the previous Schmidt–Rubin series of rifles, the K31's locking lugs lock up immediately behind the chamber. This forward positioning of the locking lugs affords several advantages. The entire action is strengthened as the lugs lock into the much thicker forward part of the receiver. Lock-up is also more precise.
The cocking piece doubles as a safety and is attached at the rear of the bolt sleeve assembly and secures the firing pin. When the cocking piece ending in a cocking ring is pulled rearward and turned horizontal, the cocking piece sear can be placed in a recessed safety slot in the bolt plug. This slot is shorter than the firing slot so the firing pin cannot protrude past the face of the bolt cylinder. Any contact with the cartridge primer is thus prevented. The safe mode also prevents the action from being cycled hence preventing the bolt from accidental opening. The operating ring is quite large, making it easy to operate with gloves. When the operating ring is in the vertical position and pulled back by cycling the action or cocking it by hand the action is ready to fire.
The Karabiner Model 1931 feeds from a detachable box magazine machined to match the cartridge for which the rifle was being chambered, that can hold up to 6 rifle cartridges. The magazine release button is an integral part of the magazine. For reloading the K31 box magazine was normally not exchanged for another magazine but a unique formed phenolic resin embedded paper stripper clip with a tinned metal edge holding six rounds was used. Whereas most chargers or stripper clips only held the rounds at the end of the cartridge cases, the K31 charger nearly covers the entire cartridge. The charger has a guide slot wide enough for a gloved thumb to force rounds down and into the magazine in one smooth motion. When the last cartridge from the magazine is fired, the follower comes up automatically during cycling locking the bolt open and preventing it from closing reminding the user the K31 needs to be reloaded.
The Karabiner Model 1931 had a two-stage trigger with a noticeable long take up before the trigger engages the sear. This feature aids in preventing premature firing during stressful (combat) situations.
The standard iron sights on a Karabiner Model 1931 are open sights that can be adjusted for both windage and elevation and have a sight radius of 568 mm (22.36 in). The rear sight is graduated from 100–1,500 m (109–1,640 yd) in 100 m (109 yd) increments. The sight line can be adjusted with a front sight adjustment tool. Moving the front post 1 mm (0.04 in) horizontally results in a 120 mm (4.72 in) shift at 300 m (328 yd). To adjust the average height of the point of impact 5 front posts ranging from 5.9 to 7.1 mm (0.23 to 0.28 in) height in 0.3 mm (0.012 in) increments are available. The change in impact height from one front sight to the next is 160 mm (6.30 in) at 300 m (328 yd). Starting at 300 meters the shooter should aim just below the bottom of the target, so that the front sight's post is out of the way. Mounting a telescopic sight conventionally is not easily done because of the design of the action, but there are specialized telescopic sight mounts available.
As the Swiss have a militia army where soldiers sometimes keep their service rifles for a lifetime and also compete with their service rifle many aftermarket sights were available: Waffenfabrik Bern made the "S" and "K" (Klammer) diopter sights, Wyss makes the "W" diopter and Furter, Haemmerli and Gruenig and Elmiger made now rare special windage and elevation fine-correctors, Sahli and many other made elevation fine correctors and these days a company by the name of Swiss Products in the USA makes a clamp-on diopter which was recently approved for use at official Swiss shooting matches.
There were three Karabiner Model 1931 variants that featured telescopic sights. These were the:
- Model 31/42, 1.8×9 telescopic sight adjustable from 100–1,000 m (109–1,094 yd) in 100 m (109 yd) increments
- Model 31/43, 2.8×14 telescopic sight adjustable from 100–700 m (109–766 yd) in 100 m (109 yd) increments
The telescopic sights of these models were made by Kern and mounted on production Karabiner Model 1931s chosen for their accuracy offset on the left side of the receiver enabeling the shooter to use the standard iron sight line.
In the 1950s an elaborate modified variant of the Karabiner Model 1931 was developed for designated marksman/sniper use. This rifle was not issued as a Model 31 variant, but as the Zielfernrohr Karabiner 55 (ZfK55) Sniper Rifle. It featured a more powerful 3.5×22 telescopic sight made by Kern adjustable from 100–800 m (109–875 yd) in 100 m (109 yd) increments. The ZfK55 weighs 6.1 kg (13 lb) empty with the telescopic sight mounted and has an overall length of 1,210 mm (47.64 in). The ZfK55 only has four small parts in common (the cocking piece, the firing pin, the firing pin spring, and the extractor) with the Karabiner Model 1931. The telescopic sight mounts are an integral part of the receiver. The 3.5×22 telescopic sight features an integral quick release mount that connects to the mounts on the left side of the receiver. The entire action of the ZfK55 is tilted at an angle of approximately 15 degrees to provide a centered position of the telescopic sight over the action and stock. The tilting of the action and magazine also provided room for the unimpaired loading and ejecting cartridges with the telescopic sight mounted. The barrel fitted to the ZfK55 is heavier than the one on the Karabiner Model 1931 and is fitted with a muzzle brake. The ZfK55 also has a half-stock with a checkered pistol grip instead of a semi-pistol grip and an underfolding integrated bipod. A total of 4,150 ZfK55s where manufactured.