The Karabiner 1931 rifle - also known by its abbreviated name, the K31 - is a Swiss military rifle produced from 1933 through 1958. All extant guns (as far as I know) were made by Waffenfabrik Bern (Weapons Factory Bern) for the Swiss Army.
The rifle is chambered for the 7.5x55mm Swiss round, also known as the 7.5 Schmidt-Rubin (after an earlier rifle which used it) or GP11 (Gewehrpatrone 11) after the Swiss military designation of the round. The K31 was never used in warfighting, seeing as it was the rifle of a neutral country. In the 1940s and 1950s, Swiss reservists (i.e. most Swiss) were issued K31s to keep in their homes as their standard rifle.
Now that it's the future, the K31 was replaced back in 1958 by more modern battle rifle and assault rifles. These guns were collected and stored in Switzerland, until the Swiss realized what most countries with surplus guns have which is that you can probably sell them to American civilians for cash. So they have.
The thing about the K31 is that it is an extremely well-made gun, built for precision and marksmanship which is unusual in a general-issue military rifle. Also, as surplus, they can be had in the U.S. for around $300 if you're lucky. Finding ammo is a bit more of a PITA, but it's doable for around $0.50/round, and the surplus ammo is also of high quality - shooters report achieving 1 MOA with unmodified surplus K31s and surplus ammunition. That means that at 100 yards, all shots land within a 1-inch circle.
The guns weigh in at 4kg when empty, and have approx. 600mm long barrels. They are fed by a detachable 6-round magazine issued with the rifle - the magazine, receiver and bolt are all serialized, and as issued all matched. They are not disposable magazines, though - the K31 was loaded using phenolic stripper clips from above, with the magazine in the rifle. Everything about it was designed to be used with thick protective clothing, because hey, it gets cold in the Alps.
Probably the most distinctive feature on the gun is its action. Although it is a bolt-action rifle, the bolt is not rotated and then slid; there is a lever on the side of the gun that is pulled straight back to eject, and straight forward to load a new round and lock the bolt. There are cam-action grooves on the bolt that unlock it when the lever is pulled. As a result, the user can keep themselves lower down when working the action. The safety is a large ring at the back of the bolt, designed to be gripped and turned by a gloved finger.