SCHMIDT – RUBIN KARABINER 1931

By Scott Stoppelman

Between the years 1933 and 1958, the final incarnation of the Schmidt-Rubin rifles that began with the M1889 Infanterie Gewehr, was built at the Eidgennossische Waffenfabrik in Bern, Switzerland. That rifle, the Karabiner 1931, or K-31 as it is commonly known, was like its predecessors – a straight-pull bolt action design that to many American shooters may seem a little odd.

The rifle’s designer, Colonel Rudolph Schmidt, offered his design to the military while employed as a weapons technical officer at the government arsenal in Bern.

The cartridge the K-31 is chambered for, the 7.5×55 Swiss, was, like the rifle itself, a final version but of a design by Eduard Rubin, that had originally been experimented with in the Swiss service rifle that predated the Schmidt-Rubin design; the 1869 Vetterli 10.4×38 rimfire rifle. The Vetterli rifle was not a strong enough design to handle the increased pressure of the newer center-fire round. The cartridge would eventually be adopted as the 7.5×53.5 for use in the M1889 rifle, the first of the Schmidt-Rubin service rifles. The round would evolve into the longer 55mm case for the later Gewher 1911 model rifle and carbine, and then continue on into the last rifle of the series, the K-31. On a historical note, in the 1930s, the Swiss Guard at the Vatican was issued one hundred K-31 rifles, making the Vatican the only other “Sovereign Nation” to be issued Schmidt-Rubin rifles.

Design

The cock-on-opening design, while it is a bolt action, is somewhat unusual in that as opposed to the more common turnbolt style action most are familiar with, this rifle is of the so called straight-pull design. This simply means that instead of lifting the bolt handle to turn the bolt lugs out of their locking recesses, the Schmidt-Rubin bolt is merely pulled straight back to accomplish the same thing. The actual turning of the lugs is accomplished by means of an actuating rod on the side of the bolt sleeve that engages a helical or spiral groove in the sleeve which forces it to rotate as it is moves back and forth. The K-31 bolt differs from earlier models in that it is shorter and has its locking lugs forward near the head of the bolt sleeve whereas the earlier models had the very long bolt with lugs nearer the center of the body. These changes in 1931, part of a modernization program for the rifle, made the new bolt both shorter and stronger. The models with the lugs near center for the earlier, lower pressure version of the 7.5 round allowed the bolt to compress and thus would not tolerate much more pressure than was present with the M90 loading. This fairly mild load used a round nose bullet of 210-grains at around 1,970 feet per second.

At the rear of the bolt is a large finger-pull ring that serves as both the safety and a manual cocking/decocking device. The safety appears to be two-position. When the ring is pulled back far enough to allow the rectangular tab to rotate and rest on the flange, the rifle will not fire, but the bolt can be operated. Rotate further clockwise and then allowing the ring to go forward in the 9 o’clock slot will put the rifle on safe and lock the bolt as well.

On the right hand side of the rifle just under the receiver opening, is a spring-loaded grooved pad bolt stop that, when depressed, drops an engagement lug and allows the bolt to be withdrawn for access to the breech.

All of the Schmidt-Rubin rifles used a detachable 12-round capacity box magazine for the first model and six for subsequent rifles. The magazine could be loaded via an unusual cardboard and metal charger that held six rounds. It could also be loaded with single rounds while in or out of the rifle. The K-31’s magazine follower is designed to stop the bolt’s forward travel when the magazine is empty as is common with many service rifles.

K-31

An example of this rifle was recently acquired at a large chain sporting goods store for a bargain price. It was a little rough around the edges as received with a bit of rust here and there. The light colored beech wood stock showed the expected dings and scratches but appeared serviceable. However, the top of the comb looked like someone had taken a rasp to it. Upon removing the bolt to inspect the bore, it was found to be incredibly dirty and, even with a bore light, evidence of rifling could barely be seen. Using a .30-06 cartridge to see if it would pass the “bullet test,” a bullet was placed into the muzzle. If the bullet disappears up to the case neck it’s likely the bore is worn either by shooting or cleaning from the muzzle too often, making potential accuracy suspect. The bullet stopped in the muzzle well before the case mouth could touch. Running a patch soaked with Hoppes #9 down the bore a number of times revealed that the bore, though it had been extremely dirty, was still in excellent shape with strong rifling and no discernable throat wear. The bolt face likewise showed little evidence of much shooting.

The rifle was completely disassembled to inspect and clean all parts of dirt, oils or grease. Disassembly is simple and done in much the same way as any number of similar military rifles. Only a slotted screwdriver is required. All serial numbers matched on this rifle including the stock and handguard. It doesn’t really matter whether all numbers match or not as long as the bolt and receiver do, as that is the most crucial fit of the rifle because of cartridge headspace. As an added bonus upon removing the buttplate for cleaning, a small plastic card with the name and other information of the last person the rifle was issued to, was found. On the other side of the card was the rifle’s serial number which also matched this rifle. There are a number of references to these I.D. cards on other shooters’ rifles on the internet, and in one previously published article, so it was intriguing to find one with this rifle.

The model K-31 has a barrel length of 25.7 inches whereas earlier models started out at 30.75 inches, then down to 23 inches for the 1911 Karabiner model to the final K-31. It is a four-groove barrel with a right hand twist of 1:10.5.

One of the more interesting features of the rifle is the drift adjustable front sight that, unlike most drift type front sights that move side to side, this one moves fore and aft in an angled dovetail groove. The rear sight is of the fairly common tangent style adjustable from 100 to 1,500 meters. It is, however, not windage adjustable as that function is performed at the front sight. Perhaps the reason for the angled dovetail on the front sight is that more left and right movement is possible within the confines of the front sight base than if using the standard right angle dovetail.

The stock has two bands to secure the forearm to the stock and, in the case of the rear band, to provide for the forward sling swivel. The front band is a little unusual in that it is actually a hinge affair that clamps to the stock. It is tightened with a screw on the other side. It also incorporates into it the bayonet lug and stacking rod. The rod is used for the same purpose as the more common stacking swivel seen on other military rifles.

With the barreled action out of the stock well done inletting can be seen. The rather large block recoil lug fits in a recess that has at its bottom a steel plate to seat the bottom of the lug metal to metal.

The trigger is another interesting piece of work. One side of the trigger housing is open allowing access to the inner workings while the other side is closed. This is a typical two-stage military type affair with a long first stage as slack, then a very crisp final stage that, on this rifle, only measures three pounds on an RCBS scale. This is an excellent trigger though set a little light by service rifle standards. It’s actually a lot lighter than most modern factory sporting rifle triggers as they leave the factory. The one piece trigger guard is large for gloved hands and fits well in the stock.

7.5×55 Swiss

The 7.5 Swiss, as it’s commonly referred to, is a fairly interesting item in itself. Considering the time period of its design, it’s fairly advanced looking with its fat body, minimum taper and relatively sharp shoulder. It very much resembles the much later .284 Winchester which, even at its introduction in 1963, was deemed by some to be a radical design. Other than caliber, the two look nearly identical side by side. Of course the other major difference between the two is that the .284 is a rebated rimless round wherein the rim is smaller than the case’s head diameter. The 7.5 Swiss has a normal rim that is slightly larger than the head diameter. Also, the headspace measurement of the 7.5 is different with the .284 having more distance between rim and shoulder. It is interesting to note that .284 brass is often used in the absence of proper 7.5×55 brass to make loads for the Swiss. Reportedly, it works just fine. However, it must be sized and fireformed. Powder capacity of the two rounds is very similar. Depending on how powder is tapped in or compressed, the two rounds will support about the same charge. In fact, as the 7.5 round is a .30 caliber, thus allowing a slightly better expansion ratio, one could surmise that in rifles of equal strength, the 7.5 cartridge may offer a slight advantage in velocities over the range of bullet weights, plus the ability to handle heavier bullets than a 7mm cartridge. In other words, it’s about the equal of the .30-06 which says a lot about its capability. That’s not to say it’s safe to “hoss it up” in the Swiss action. While the Schmidt-Rubin design is not a weak one, it likely can’t compete with a modern turnbolt when it comes to handling pressure. In Frank Barnes’ book Cartridges of the World, he states that it can be expected to give performance similar to the .308 Winchester. It’s likely he means (in) the Swiss rifle. If the two cases are set side by side, the 7.5 is noticeably larger in every dimension, except caliber. Now that boxer primed brass is available from Hornady and others, the 7.5 Swiss would make an excellent candidate for an accurate non-magnum sporter using a modern bolt action.

One of the older military loadings with the 190-grain bullet was, according to Barnes, loaded to about 37,000 psi. The later loading with the 174-grain spitzer is reportedly loaded to about 45,500 psi. Neither of these is much by today’s standard. Muzzle velocity is stated as around 2,650 fps with the 174-grain load. Two rounds of this load were clocked at 2,500 and 2,516 fps on my chronograph.

Bolt Takedown

It is not certain that this is the exact official method for bolt takedown, but it works. With the action cocked, remove the bolt from the receiver. Pull back on the safety/cocking piece ring until the small rectangular tab clears the flange, then rotate the ring clockwise to between seven and eight o’clock to allow the tab to rest on the flange. This allows the actuating rod to be removed from the bolt sleeve. Rotate the sleeve until it disengages itself from the rear portion of the bolt and flange. Tip the sleeve down to allow the inner bolt to fall out into the hand. To remove the tip of the two piece firing pin assembly, first decock the firing pin by turning the ring to allow the tab to go all the way forward in the straight slot. This releases some of the strain on the spring. Pull back on the spring with one hand while with the other hand, move the tip off laterally, and then remove the spring if needed. To complete the disassembly remove the ring/stem from the flanged part of the bolt. Reassembly is in reverse order. This is all accomplished without any tools: a plus for a service rifle.

The extractor, which resides on the top of the bolt just past the 12 o’clock position, is not large but is apparently adequate. Ejection is via a substantial folding blade in the bottom of the receiver just behind the magazine well. The bolt face is slotted for the ejector but the locking lug isn’t as in some designs.

Shooting the K-31

Only one word is needed to describe the shooting and handling qualities of the K-31 rifle: outstanding! The Swiss gun lived up to its reputation as an accurate rifle. Shooting 174-grain ball ammunition from a portable rest at about 85 yards and using a makeshift target (bucket bottom), the rifle’s first six shots went into a group of about two and a half inches. A subsequent three shot group went one inch and another group put four of five into one inch as well. The lone flyer was shooter error getting used to the light trigger. Groups were well centered left to right but all were about five inches above point of aim at that distance. This seems to be typical of battle sights that are commonly set for 300 yards. The u-notch rear sight provided a good sight picture with the blade front.

The rifle fed smoothly with no misfires and extracted and ejected perfectly. I always knew where the empties were because they went straight up and then bounced off my noggin upon descent. How high they fly is determined by how much vigor is employed in bolt operation. For the handloader, the bolt can be eased back slowly for easy removal of cases from the open action. I really have no criticism of the rifle other than the trigger being a little light for a service rifle. A three pound pull is more appropriate for a single-stage hunting or match rifle trigger.

As a combat rifle we may never know for sure how it would have worked, or if it was as good as other proven designs like the Mauser, Enfield, or Springfield. Most bolt action service rifles exhibit a fair amount of play in operation, to allow for mud and debris that invariably finds its way into guns in battle, and still remain functional. When the bolt is run back and forth in the Swiss, contact is near metal to metal between bolt and receiver opening, and there is but little play. Still, military training usually will point out potential problems and judging from the condition of the stocks on Swiss rifles observed, many appear to have been used for purposes other than target shooting. At any rate, the Swiss stayed with the same basic design for well over a half century so it would seem they had a fair amount of confidence in the design.