Operating Systems 201: Telescoping Bolts

IMG_0947

Previously in TFB’s series on weapon operating mechanisms, we examined both the closed-bolt blowback system and the open-bolt API blowback system, two very close relatives that share a common feature: Closure of the breech through the inertia of the breechblock mass alone. What this means for small arms designers is that they must engineer a system where the breechblock is large enough to have sufficient mass, while having enough room to reciprocate the distance necessary for reliable ejection and feeding. In a standard submachine gun design using the API blowback system, for example, the breechblock is configured behind the breech end of the barrel, so that it can reciprocate. This is an easy design to engineer, and can be made simply and easily.

bolt-face

You can see the form of a conventional submachine gun breechblock in this bolt from a Blyskawica SMG, made covertly in shops in Poland during the Second World War, as part of the Polish fight against the Nazis. The round recess closest to the camera is the breech face, and virtually all of the bolt’s mass is located behind it. Image courtesy of Leszek Erenfeicht

 

However, beginning in the 1940s, there emerged a demand for submachine guns to be made smaller and more compact, to allow their use by echelon troops, police, and special forces. This pressure spurred the invention of what’s called the “telescoped” or “telescoping” bolt configuration, which differs from the traditional layout in having a large portion of its mass in front of the breech, rather than behind it. This allows the bolt’s mass to be stored above the barrel, greatly reducing the length of the receiver and making for a much more compact weapon, while still allowing for economical manufacture and the use of unlocked blowback operation (either pure or API).

Rr0JjSl

This Walther MPL bolt exploits the telescoping principle. You can see the breechface as the rightmost edge of the rectangular section below round main body. Much of the bolt’s mass is laid out forward and above the breechface.

 

Weapons using telescoped bolts enjoy a huge benefit in compactness versus their conventional counterparts, but the older, simpler style still lives on, especially in improvised and covertly made weapons that can be found all over the world.



Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. In addition to contributing to The Firearm Blog, he runs 196,800 Revolutions Per Minute, a blog devoted to modern small arms design and theory. He can be reached via email at nathaniel.f@staff.thefirearmblog.com.


Advertisement

  • Al Wise

    Wouldn’t an article about telescoping bolts benefit from at least one photo of one?

  • Andrew Thomas

    besides the kel tec cmr-30 are there any current production firearms using a telescoping bolt?

    • Jonathan Ferguson

      The Uzi is still in production. Depending upon definition, there are various designs with substantial bolt carrier mass in front of the bolt itself.

    • Andrew

      All the decedents of the M-10 / MAC-10 (and Uzi models, as has been mentioned) come to mind.

    • AHill

      Vz 61 is still in production.

  • PK

    I’m surprised you didn’t mention the Sa vz. 23, being that it was the first.

    • The Winchester selfloaders preceded it.

      • PK

        Well, today I learned! Thanks for the correction. I had my mind firmly on SMGs in reference to telescoping bolts, but you’re right.

  • Drew Coleman

    Doesn’t the UZI have a telescoping bolt?

    • Jonathan Ferguson

      Yup, hence the picture at the top of the article.

  • Joe

    Are there any rifles that utilize a telescoping bolt?
    Seems like a potential way to shorten the excessive length of pull on bullpup designs.

    • noob

      Bullpup rifles use a locked action which means bolt mass can be kept to a minimum. The bolts are already as small as they can be in a bullpup rifle.

      A bullpup blowback pistol calibre carbine? Now there a telescoping bolt will be interesting

      • Kivaari

        The CX4 is a essentially a bullpup. The magazine is in the grip, like an Uzi. The hammer is well behind the magazine. I like ’em.

    • Yes, several. The Beretta CX4 Storm, for one.

      • Kivaari

        I recently bought a CX4. Except for a heavy trigger on my particular rifle, they are very nicely made. The bolt assembly is beautifully made.

  • Jason Nasca

    Isn’t a pistol slide a telescoping bolt?

  • Alexandru Ianu

    Wouldn’t the Winchester 1905/07/10 be the first gun to utilise a telescoping bolt?

    • It would be one of the first long guns, yes. If you think about it, though, pistol slides are basically telescoped bolts, too!

  • Indeed. As we’ll discuss later, mass is still a very important quantity even for locked breech forms of operation.

  • spotr

    Telescoping bolt – simplified diagram.

  • Kivaari

    How would you describe the MP38/40, where it telescopes from the rear. 3-piece from memory.

    • Conventional non-telescoping. The telescoping hydraulic guide rod on the MP.40, while admittedly confusing, isn’t the same thing as a telescoping bolt.

      • Kivaari

        OK. It’s how I viewed it since a kid. But, I get your point. I had a Standard Uzi and Mini Uzi. I like ’em, even though they aren’t as nice as the HK MP5.