K31

The Karabiner Model 1931 (K31) is a magazine-fed, straight-pull bolt action rifle. It was the standard issue rifle of the Swiss armed forces from 1933 until 1958 though examples remained in service into the 1970s. It has a 6-round removable magazine, and is chambered for the 7.5×55mm Swiss Gewehrpatrone 1911 or GP 11, a cartridge with ballistic qualities similar to the 7.62×51mm NATO/.308 Winchester cartridge. Each rifle included a 6-round detachable box magazine with matching stamped serial number. A charger is used to load the magazine from the top of the receiver.

The Karabiner Model 1931 replaced both the Model 1911 rifle and carbine and was gradually replaced by the Stgw 57 from 1958 onwards.

Although the K31 is a straight-pull carbine broadly based on previous Swiss "Schmidt–Rubin" service rifles and carbines, the K31 was not designed by Colonel Rudolf Schmidt (1832–1898) as he was not alive in 1931 to do so.^[1][2] Mechanical engineer Eduard Rubin (1846–1920) was the designer of the 7.5×55mm Swiss ammunition previous Swiss service rifles and the K31 are chambered for. The Karabiner Model 1931 was a new design by the Eidgenössische Waffenfabrik in Bern, Switzerland under Colonel Adolf Furrer (1873–1958). The first 200 K31s were made in May 1931 for troop trials (serials 500,001 – 500,200), thus the model number of 1931.

Design details

K31's straight-pull bolt action bolt group

Features

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 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 and quality for a service rifle. 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.^[3] 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.

Safety

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.

Ammunition feeding

Stripper clip inserted in K31

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. 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 K31 needs to be reloaded.

Trigger

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.

Sights

Tangent rear sight.

Front sight post.

K31 open sights arrangement.

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).

K31 6 o'clock hold for 300 m (328 yd) and center hold for 200 m (219 yd)

The standard K31 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 K31, marksmanship training and competitions. 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).

Mounting a telescopic sight conventionally is not easily done because of the design of the action, but there are specialized telescopic sight mounts available.^[5]

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.

Modifications history

During its production run there were several modifications tested, rejected and made to the K31.

Accessories

Karabiner Model 1931s were issued with slings, muzzle caps, knife-type detachable bayonets, cleaning kits and carrying pouches for stripper clips.

Variants

Zfk 31-43 rifle

ZfK55 rifle and telescopic sight storage container

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 room for the unimpaired loading and ejecting cartridges with the telescopic sight mounted. The tilting of the action and magazine also provided a centered position of the telescopic sight over the action and stock at the cost of widening the rifle somewhat. 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.

Poor stock condition

The poor condition of many stocks was caused mostly by the wearing of crampons [ice-cleats that project not only from the sole but the sides of the heels] worn over hobnail boots and rifle drills that were common. The military habit of stacking rifles in threes - often in the snow - also contributes to the 'ragged' appearance of the end of the butt. Postwar beech stocks are more affected than the older walnut ones. Walnut stocks - the material of choice prior to 1946 - were treated with linseed oil and later beech stocks got a shellac protective layer that easily dissolves in alcohol for (arsenal) repairs.

Availability

As of 2010, the Swiss arsenals are long sold out and the rifles now available for sale from military surplus vendors in countries around the world are ex-Swiss-civilian owned rifles. The stocks are usually in average condition, but the barrel and bolt assembly are usually in very good condition because the Swiss used a special gun grease known as Waffenfett instead of gun cleaning oil, and the issued ammunition was non-corrosive. Some K31s can be found with "trooper tags" underneath the steel butt plate at the rear end of the stock, showing its former Swiss government user. Many collectors of the K31 recovered a small tag of plasticized paper containing the military unit, name and address and pension number of the Swiss citizen to whom the rifle was issued. In some cases, collectors have used the information to contact the previous users, and have recounted the details of those encounters on a variety of collector's web forums.

Civilian use

In Switzerland the Karabiner Model 1931 is like other (ex) Swiss service rifles used for target shooting matches. Recreational practice with guns is a popular form of recreation, and is encouraged by the government, particularly for the members of the militia. Typical Swiss rifle shooting (Eidgenössisches Feldschiessen) is done with an (ex) Swiss service rifle at a range of 300 m (328 yd), prone. For this the standard iron sights can be replaced by target shooting diopter and globe sight sighting lines. In other countries the K31 can often be used in vintage military service rifle matches. Clamp-on sighting options for competition diopter style sights and telescopic sights make it easier to mount more precise aiming means than the standard factory tangent iron sights on the receiver. Many competition shooters are able to achieve 1 MOA shooting groups with unmodified K31s with the factory tangent iron sight line.

To celebrate its introduction in the Swiss armed forces a small commemorative batch of Karabiner Model 1931s was produced 80 years later.

Gallery

• 
  K31 bayonet M1918 and its frog.
• 
  K31 box magazine loaded with inert 7.5×55mm Swiss training cartridge.
• 
  K31 Phenolic stripper clips with GP11 ball cartridges.
• 
  K31 muzzle caps for protecting the muzzle.
• 
  K31 with W+F diopter rear sight for match shooting.
• 
  K31 with W+F globe front sight for match shooting.
• 
  K31 with W+F match shooting sight line.
• 
  K31 tangent rear sight element with windage and elevation fine-correctors made by Furter.
• 
  K31 disassembled bolt assembly.
• 
  Single K31 bolt

Media

• A video of the K31 straight pull bolt in action: Media:k31.ogg
• In the film Shining Through, a Swiss border guard, with his K31, shot a German sniper firing at Ed and Linda as they were crossing over the Swiss border.^[6]

Users

Country	Organization name	Quantity	Date	Reference
Switzerland	Swiss Armed Forces	528,230	1933 to 1958	[7]
Vatican City	Pontifical Swiss Guard	100	1955	[8]

See also

• Antique gun
• M1895 Lee Navy - An American straight-pull bolt-action rifle
• Ross rifle - An ill-fated Canadian straight-pull bolt-action rifle
• Schmidt–Rubin
• Steyr-Mannlicher M1895 - An Austrian straight-pull bolt-action rifle

References

1. ↑ Bobinson, Holt (2008). "The model 1911 Schmidt Rubin: the other Switzer". Guns Magazine. 
2. ↑ The Gun Digest Book of Firearms Assembly/Disassembly: Centerfire Rifles, Volume 4 by J. B. Wood. Published by Krause Publications, 2003. ISBN 978-0-87349-631-5
3. ↑ K31 manual, see page 66.
4. ↑ K31 manual, see page 56, figures 32 & 33 for aiming points.
5. ↑ SCHMIDT-RUBIN K31 SCOPE MOUNT at Brownells
6. ↑ http://www.imfdb.org/index.php/Shining_Through
7. ↑ Swiss Karabiner 1931
8. ↑ Swiss Karabiner 1931

External links

Wikimedia Commons has media related to Schmidt-Rubin M1931.

• Swiss Rifles
• The Swiss Rifles Message Board
• Surplusrifle.com's articles on the K31
• Modern Firearms entry on the K31
• chuckhawks.com article on the K31
• Manufacture Dates of Swiss Schmidt–Rubin Rifles
• Site and tool to determine manufacture year of Swiss Schmidt–Rubin Rifles
• Webbased Tool to find the manufacture year based on the manufacturer serial number (German Website)
• The K31 Swiss Service Rifle
• Swiss Karabiner 1931