A look inside the Swiss K31 Bolt

K31 profiles

Those beautiful K31 bolts are incredibly smooth compared to other straight pull rifles.  They’re also much simpler than the competition.  This next paragraph is going to have a lot of words about just how this works but hopefully the animation I whipped up will be clear enough on its own.

k31-bolt-animation

Swiss straight pull rifles feature a bolt body with extractor that does not rotate during operation. Instead a collar, with locking lugs, is wrapped around the bolt body.  This is essentially a cylindrical bearing with a helical channel in one side.  The bolt handle has a rail attached, with a lug that is round in the middle and square at the end.  When retracted, the bolt handle’s square lug compresses the striker spring and its round center slides in the helical channel of the collar, turning it to unlock the lugs.  The bolt handle’s lug would then set into a notch in the collar at the base of the helical channel, locking the striker back and therefor cocking the action on open.  On the return stroke the striker would catch the trigger sear and remained compressed as the bolt handle lug, traveling in the helical channel, turned the bolt body to lock in the receiver.  The large ring at the back of the bolt is a part of the striker.  Pulling this back and rotating it 90 degrees into the left slot essentially de-cocks the gun and prevents the firing pin from reaching the cartridge.

More about this particular rifle can be found at Candrsenal.

Related

Othais

Othais is practically useless with modern firearms. That’s OK though, because he specializes in Curio and Relic military pieces and has agreed to decorate The Firearm Blog with a little history. He maintains his own site, C&Rsenal, with the help of his friends and the collector community.


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  • Clint Notestine

    I almost bought one years ago but ended up going with a No. 4 Mk.1. Was like $100 now its more like $250-300

    • DiverEngrSL17K

      Still an excellent price ( in fact, I think it’s a steal ) , considering what you’re getting ( as long as you do your research and short-listing properly ).

  • seldomwright

    These are amazingly overlooked and accurate rifles. A company called Mojo Sights makes replacement front and rear peep sights that make these much more enjoyable to shoot without going the more involved scope mount route.

  • Herr Schakal

    Proud to be the owner of a K31. Made in 1954, shoots like a dream.

  • Nicholas Mew

    Just imagine if the Swiss start producing them again as sporter rifles.

    • Cornelius Carroll

      I’d buy one in a second if it was priced reasonably (~$500), had a quality barrel, and was chambered in something readily available (say 7.62×51… 7.62×54 would be cool too though).

      • Raven

        According to something I’ve read, a new-production K31 built to the same standards and materials would be more like $1300US just to manufacture.

        • SentMKG

          I believe it with how well they were made. I own a few variants, one is 106 years old, and have yet to get the K31 but have handled have been great. They are manufactured to amazing standards.

          They have the best trigger for a standard issue rifle I think ever. Good powerful round. Love that they used match grade ammo so it has zero rifling damage or anything else to worry about.

          • SweatTowel

            I highly doubt that you have a K31 that’s 106 years old.

          • RocketScientist

            Ummm, he said he had a few VARIANTS, but doesn’t have a K31 yet. It was pretty clear he meant he had a schmidt-rubin straight-pull of some sort that was 106 yrs old, which is certainly possible, considering they’ve been making variations of that design for about 125 year or so.

        • Ben 10

          they can always use cheaper materials and more modern manufacturing methods like using stamped sheet metal receivers to lower the cost. the 6.5×55 swedish is not a very high pressure cartridge.

          • DiverEngrSL17K

            The K-31 fires the high-pressure Swiss 7.5mm x 55 cartridge, which is about equivalent to the 7.62mm x 51 NATO / .308 Winchester, Russian 7.62mm x 54R or .30-06 cartridges in terms of approximate performance and chamber pressures. I don’t think any K-31′s were ever made in calibers other than 7.5mm x 55. If any K-31 based rifles exist in alternative calibers, they are definitely not original K-31′s, but are probably specialized conversions. The K-31 should not be confused with the earlier Swiss Schmidt-Rubin M1896/11 and M1911 rifles, which also used a straight-pull action. Some of the latter were imported into the U.S. in the 1960′s and 1970′s as “sporter” rifles or were modified as such over here for sale mostly to hunters.

            One still sees on-line articles, even some by respected firearms authorities, that refer to the K-31 as a Schmidt-Rubin. This is not correct. The K-31 was a completely new design produced by a team, led by Colonel Furrer, at Eidgenossische Waffenfabrik of Bern, and was intended strictly for military service.

            On another note, the 6.5mm x 55 Swedish cartridge is definitely still a high-pressure cartridge, having a maximum C.I.P. rating of 380.00 MPa ( 55,114 psi ) piezo pressure, and a SAAMI Maximum Average Pressure of 351.6 MPa ( 51,000 psi ). This might not be as high as the operating pressures for some other rounds, but it hardly qualifies as anything less than a high-pressure rating and is quite close to the rating ( within 2% for C.I.P. ) for 7.62mm x 54R, which no-one in their right minds would consider as anything less than a very powerful, high-pressure cartridge.

            By the way, C.I.P.-approved firearms chambered in 6.5mm Swedish still have to withstand proof loadings of 125% of the typical maximum, which translates to 475.00 MPa or 68,893 psi piezo pressure.

      • DiverEngrSL17K

        7.5mm x 55 Swiss ammunition ( both military surplus GP11 FMJ / ball and aftermarket ammunition, such as PPU ) is not difficult to obtain, provided you do your research and compile a good list of vendors you can go to. Cost-wise, it isn’t any more expensive than equivalent 7.62mm x 51 or .308 when they are discounted for sale.

        Unless you absolutely insist on a “sporter” version, standard military K-31′s in excellent mechanical and cosmetic condition are available for less than $500, again if you do your research and shop around on web sites such as http://www.simpsonltd.com or http://www.aimsurplus.com , to name just two better-known ones.

        Frankly, I’m not quite sure what your definition of a “quality” barrel is — the barrel on the military-issue K-31 is certainly still first-rate by any standard.

  • Lance

    One of the best Swiss inventions since the Coo Coo Clock!

    • DiverEngrSL17K

      Don’t forget Rolex, Omega and Wenger watches, Swiss-made tritium vials for almost every conceivable night-vision and night-sight application one can think of, almost every SIG military-grade weapon ever made, Wartsila-Sulzer diesel engines ( with due acknowledgement to the Finnish half of the partnership ), Swiss chocolate and a whole plethora of other items too numerous to list. And, most importantly, many truly unique and wonderfully open-minded people.

  • Rob in Katy

    I have a buddy that let me shoot his, not sure what caliber it was, but that bolt was smooth – the thing kicked like a mule! But dang the bolt was smoooth, not sure that it made up for the kick on my old shoulders.

    • sturm44
    • DiverEngrSL17K

      True military-issue K-31′s were only made in one caliber — 7.5mm x 55 Swiss, similar to Russian 7.62mm x 54R, 7.62mm x 51 NATO/.308 Winchester, or .30-06 Springfield. Coupled with that old-style steel buttplate and hardwood stock, I’m not surprised it felt as if it “kicked like a mule”.

  • Exoskeleton

    Great animation, thanks for the effort!

  • John Bear Ross

    It’s neat when they still have the ID paper under the buttplate. I love these butter-smooth rifles.
    Best,
    JBR

  • vitor roma

    I wonder why we dont see more rifles with this kind of bolt. It is considerably faster to reload the next round compared to traditional manually operated bolts without any considerable disadvantage.

    • Raven

      Price and laziness on the part of manufacturers.

    • tacticaltshirts.com

      Blaser currently offers a straight-pull design. It’s use a collet bushing as opposed to a rotating bolt, and it’s wicked fast. I wasn’t a believer at first, but over time I have really come to appreciate how light compact that action is. –TTR

  • dhfshfsghhf

    Shut up and take my money…
    Wait, i already have a K-31…

    Shut up and take my money!

  • Cornelius Carroll

    Absolutely love straight pulls. Wish there were more of them out there.

    • dieks62

      Actually; from memory there are a few ‘clones’ out there. However: I know most of you would not like me say this, but the K31′s or the earlier Smith-Rubin’s are no straight-pulls rifles in the true technical gunmaking-design terms of the word! Straight-pull rifle means no turning bolt-heads to lock-up or rotating bolt-handles from the bolt-axis! The Merkel K-1 and both Blaser R93/R8 are only ‘partial’ ‘straight-pulls’! The only ‘true’ one in my book is the German mfg’erd Heym SR30!

  • Vhyrus

    sig makes a hardcore sniper rifle in 308 and 338 with a straight pull bolt called the tac2. very expensive but the action is amazing.

    • Mark

      It’s actually made by Blaser and marketed by Sig. It’s a straight pull with a pivoting bolt handle that rotates toward you (the shooter) to unlock the action before the action slides open and away from you to lock the action after it closes. Once you get used to it it’s a much easier system to use than a conventional bolt action.

  • Peter Balzer

    the pics give me the creeps. with the cocking ring in that position, you’re looking at a cocked and off-safed rifle. every swiss rangemaster would kick you out, based on this. know your weapon(s) and be safe!!

  • legersois

    A lot of shooter use this rifle in France, for most part 200 yards events. Ammo are available, and its really, really accurate !

  • mechamaster

    It is possible to do ‘mad minute’ firing with K 31 rifle like Lee-Enfield ? ( just a little
    curiosity )

  • DiverEngrSL17K

    Othais, thanks once again for a great article about a great rifle. I have two K-31′s, and will probably add another to my collection in the next few months. And for those who think that Swiss 7.5mm x 55 ammunition is hard to come by, that is far from the truth. Surplus Swiss military GP11 ammunition is usually readily available at a reasonable cost ( about the same as 7.62mm x 51 NATO surplus when on sale at discounted prices ) from one source or the other if you do your research and build a good short-list of preferred vendors. Outside of that, manufacturers such as PPU ( Prvi Partizan ) also manufacture a variety of rounds for this and other 7.5mm Swiss rifles.

    Insofar as consistent accuracy and reliability are concerned, there are few full-caliber rifles, military or civilian, that are a match for a standard K-31, let alone an accurized version.

    Truly an under-rated yet technically outstanding firearm.