K31

Compared to the previous Schmidt–Rubin series Model 1911 rifle and carbine, the Karabiner Modell 31 bolt and receiver were significantly shortened, allowing for a rifle length barrel and sight radius, without increasing the overall length of the Model 1911 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 Modell 31 barrel has 4 rifling 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 recoil lug, with the second attaching to the tang. This allowed the Swiss to eliminate the aluminium barrel mounting collar used in the Schmidt-Rubin series. The trigger was also redesigned.

Karabiner Modell 31s are noted for their excellent accuracy and quality for a service rifle. The Swiss armed forces considered individual marksmanship to be of utmost importance. Therefore, the K. 31/Kar. 31 was made to tight tolerances and excellent overall craftsmanship. According to the Swiss Military manual for the Karabiner Modell 31 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.[3] The employed circular error probable method cannot be converted and is not comparable to US military methods for determining rifle accuracy. 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). When the R₅₀ results are doubled the hit probability increases to 93.7%.

The Karabiner Modell 31 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 K. 31/Kar. 31'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.

Stripper clip inserted in K. 31/Kar. 31

The Karabiner Modell 31 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 K. 31/Kar. 31 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 K. 31/Kar. 31 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. The chargers or stripper clips have to be manually removed before the bolt can be closed. 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 K. 31/Kar. 31 needs to be reloaded.

The Karabiner Modell 31 has 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.

Tangent rear sight.

Front sight post.

K31 open sights arrangement.

The standard iron sights on a Karabiner Modell 31 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).

K. 31/Kar. 31 6 o'clock hold for 300 m (328 yd) and center hold for 200 m (219 yd)

The standard K. 31/Kar. 31 iron sight line elevation concept is somewhat unconventional and designed for center hold (point of aim = point of impact) at the 100 m (109 yd) and 200 m (219 yd) meter settings with GP11 ammunition. Starting at 300 m (328 yd) and more distant ranges the shooter should aim below the bottom of the target, so that the front sight's post is just out of the way.[4] The 6 o'clock hold is intended for target shooting at 300 m (328 yd), meaning the sight line is designed to let GP11 ammunition hit 30 cm (11.8 in) over the point of aim on a 60 cm (23.6 in) diameter bulls eye the Swiss military and shooting clubs used for sighting in the K. 31/Kar. 31, marksmanship training and competitions. A 6 o'clock hold is only good for a known target size at a known distance and will not hold zero without user adjustment if these factors are varied.[5] Combined with GP11 ammunition the 300 m (328 yd) and 400 m (437 yd) settings can alternatively be used as center holds for 400 m (437 yd) and 500 m (547 yd).

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 & Elmiger made 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 United States makes a clamp-on diopter which was recently approved for use at official Swiss shooting matches.

During its production run there were several modifications tested, rejected and made to the K. 31/Kar. 31.

Karabiner Modell 31s were issued with slings, muzzle caps, knife-type detachable bayonets, cleaning kits and carrying pouches for stripper clips.