The Karabiner 31 (K31 for short) is a Swiss straight-pull bolt action carbine. The standard Swiss service rifle from 1933, it remained in service with the Swiss Armed Forces until 1958, although some served until the 1970s. K31s are also noted for their excellent accuracy and construction.
The K31 was the first major redesign of the Schmidt-Rubin rifle; despite it being quite similar to the Schmidt-Rubin design, it was not designed by Colonel Rudolf Schmidt as he had long been dead by then. Instead, the design of the K31 was overseen by Colonel Adolf Furrer.
The weapon was noted to have incorporated improvements over the Schmidt-Rubin, such as a rear sight graduated down to 100 metres (330 feet; 110 yards). The first 20 prototypes were delivered to the Swiss Army for tests in 1932. The rifle was put into general issue on June 16th 1933 and discontinued in 1958, when it was replaced by the Stgw 57.
As mentioned above, the K31 was the first major redesign of the Schmidt-Rubin rifle; the main difference between both weapons is a significantly shortened receiver and bolt. The bolt, while based on what was used in the Schmidt-Rubin, was significantly shortened to about half the length and was also completely revised; this allowed the rifle to have a significantly shorter receiver while having a slightly longer barrel length, along with making the rifle stronger and less expensive to manufacture. What was the bolt sleeve on the Schmidt-Rubin became essentially the bolt on the K31, with lugs at the front end locking into the chamber; this helped in shortening the bolt.
The cocking piece is the distinctive ring on the rear of the bolt and serves two functions; not only does it ready the weapon for firing, it also acts as the safety. By pulling the ring back and turning it horizontally, the weapon is rendered safe. Simply pulling back the cocking ring also primes the firing pin, allowing it to fly forward when the trigger is pulled. The action can be disassembled without the use of tools in a short time.
The K31's trigger is noted as having a rather long take up before the sear is engaged; this is due to the trigger being a two-stage trigger. The use of a two-stage trigger acts as another safety feature in that it acts as an aid to prevent premature firing during "stressful situations" such as during combat.
As mentioned above, the rifle is a straight-pull bolt action rifle, which means that the bolt only needs to be racked back and forth to chamber a round, with no need to rotate the bolt to unlock it. As the locking sleeves rotate, the locking lugs exit the locking lug slots in the receiver and stop rotating at the entrance of the locking lug guide grooves that run along the length of the inside of the receiver; the bolt is, in this position, unlocked and may be pulled freely to the rear up to the bolt stop. At the same time, the extractor pulls out an empty case and the case is ejected at the point where it hits the ejector.
As the bolt is pushed forward, the locking sleeve turns slightly as guided by the angled surfaces on the locking lugs; this same movement frees the cam follower pin from the aft pocket in the helical slot in the locking sleeve, with the lip of the trigger sear restraining the cocking piece sear. The firing pin spring is remains compressed in this fashion. As the cam follower place is pushed further forward, the cam follower pin, guided by the helical slot in the locking sleeve, turns the locking sleeve. The locking lugs then turn into the locking lug slots in the receiver and the bolt is then considered locked. A bolt not locked completely is said to expose the shooter to the risk of accident and can also possibly damage the weapon's cam follower pin.
A rather unique feature of the K31 was that the bolt could not be closed on an empty magazine as it would be stopped by the magazine follower. The K31's iron sights are open sights which can be adjustable for both windage and elevation. The front sight is notably improved over the Schmidt-Rubin with two "ears" on the side of the sight and could be adjusted with tools; the rear sight is graduated from 100–1,600 metres (330–5,250 feet; 110–1,750 yards). The weapon has four-groove rifling in a free-floating barrel with a 270 mm (11 in) twist rate.
The weapon uses six-round detachable box magazines, although it was usually loaded with loose rounds or stripper clips; the weapon's stripper clips were made of a unique formed phenolic resin-embedded paper, with the clip covering almost the entire cartridge. The magazine release button is located on the magazine itself. The rifle was issued with slings, muzzle caps, detachable bayonets and other peripherals.
Disassembling the weapon required its user to remove the magazine, followed by the bolt (and disassembling it if necessary) and then the weapon can be disassembled. Many modifications were made over the K31's service life, such as the lightening of the firing pin in 1934, making the receiver and magazine out of hardened steel in 1935 and 1936 respectively and replacing the walnut wood stocks with beech wood stocks starting from rifle serial number 868,901 (serial numbers started at 500,001) in 1946. In 1944, chromoly steel was used in place of chromium nickel steel on some parts due to supply shortages; this proved unsuccessful.
 Fitted with standard barrel and blade front sights. Optic is noted to work very similarly to that of a periscope. Known variously as the K31/42 or the ZfK 31/42.
 Fitted with standard barrel and blade front sights. Optic is noted to work very similarly to that of a periscope. Apparently produced concurrently with the K31/42. Known variously as the K31/43 or the ZfK 31/43.
- Main article: ZfK 55
- The K31 is sometimes considered to be the last military bolt-action rifle to be designed.
- ↑ 1.0 1.1 https://www.forgottenweapons.com/swiss-straight-pull-first-impressions/
- ↑ 2.0 2.1 2.2 https://www.fieldandstream.com/swiss-k31
- ↑ 3.0 3.1 Illustrated World Encyclopedia of Guns, 2011, W. Fowler, P. Sweeney, p. 135
- ↑ Jane's Guns Recognition Guide, 2008 edition, Richard D. Jones, Andrew White, p. 251
- ↑ 5.0 5.1 http://www.swissrifles.com/sr/english_k11_k31_manual.pdf
- ↑ 6.0 6.1 6.2 6.3 6.4 https://surplused.com/index.php/2014/06/12/a-quick-and-dirty-guide-swiss-straight-pull-service-rifles/
- ↑ https://www.rockislandauction.com/detail/73/474/swiss-k3142-rifle-75x55mm
- ↑ https://www.icollector.com/Desirable-Early-Swiss-K31-43-Straight-Pull-Sniper-Rifle-with-Side-Mounted-Sniper-Scope_i17060985
- ↑ http://www.swissrifles.com/sr/