When the time came for the Swiss army to replace their black-powder Vetterli rifles, they cast about for only a few years before settling on a design by Col. Rudolph Schmidt, working out of Bern.
He had come up with a straight-pull bolt action design in which the bolt body itself was nested inside an outer sleeve. The sleeve carried the lugs which locked into mortises in the receiver body, and was rotated through ninety degrees when the bolt handle was pulled to the rear, by means of a lug on the operating rod acting on a helical track in the bolt sleeve.
The earliest Schmidt rifles were elaborate pieces of machinery; every part of the bolt assembly, from the cocking ring to the operating rod, was machined from steel forgings. For whatever reason, Schmidt had the actual bolt body itself projecting far forward from the encircling sleeve, while the locking lugs were at the extreme rear end. This resulted in the tubular machined steel receiver of the rifle being extremely bulky and heavy when compared to the various other rifle designs emerging at the same time, as well as requiring a fair amount of time and ordnance steel to manufacture; two strikes against a weapon from the standpoint of any bureaucracy.
Further, the breechface was the better part of a foot from the locking lugs. When the catridge was fired, a force of several tons was exerted straight back against the column of the bolt (know as "bolt face thrust"), and that long, thin column of steel was only supported by those two lugs at its rear end. This could lead to compression, flexing, and premature wear or failure of the action.
As higher pressure cartridges were introduced, the Swiss redesigned the rifle by moving the lugs towards the front of the bolt sleeve. This made the arrangement much sturdier and allowed the use of modern ammunition. They also lightened the rifle's receiver by milling several lightening grooves in it, and made it less bulky by going to a smaller-capacity magazine.
However, the basic layout of the long bolt protruding forward from its sleeve remained, and despite the lightening cuts, the receiver was still longer, heavier, and much more difficult to produce than its contemporaries. This solution, finalized in 1911, could be no more than a stopgap.
In the 1930s, Switzerland introduced a new service weapon. Following the trail blazed by the Americans and the British in dispensing with two different length weapons, they standardised on a "carbine" length weapon for everyone.The new rifle had a much shortened action, the forward extension of the bolt body having been "telescoped" back into the sleeve. Even though it was roughly the same overall length as the earlier Schmidt K.11 cavalry carbine, the K.31's barrel was longer, due to the length saved in the receiver. Manufacture was further simplified by doing away with the complexly-shaped operating rod and going to a simple, flat piece. All in all, it was a far more practical weapon for mass production.
Despite these shortcuts, the K.31 retained a reputation for fit, finish, and accuracy that lived up to the legacy of its Schmidt-designed forebears.