Is it a good idea?

No. A very definitive no.

However, that's not stopping a young aristocratic terrestrial cuttlefish (long story) that lives on this world's equivalent of a highly mountainous and densely forested version of Australia on steroids from attempting to make one! That, and to make sure he doesn't lose the wager he made with the heiress of a rival house on whether or not he could do it.

Now, with the reasons why anyone would attempt such a thing (since if you wanted to just make it portable, folding has been proven and has been done many multiple times over), let's ask the question:

What sort of mechanism could a telescoping barrel operate on?


  • The barrel has to extend over a preexisting barrel that then physically extends the length of the barrel. Thus extending the duration a bullet spends accelerating in the barrel.
  • Should, in theory, be reliable and sturdy (now isn't that an oxymoron) enough to be implemented; assuming the technology is a available. Speaking of which...
  • The cephalopods in this setting a very advanced compared to us, so our young aristocrat has access to advanced diamondoid's, corundumoid, future alloys with favorable strength to weight rations, compact power storage, strong mechanical and miniaturized motors etc, etc.
  • Have access to advanced manufacturing techniques, thus manufacture or parts that would be impossible to be made traditionally are not a concern.

Now, let's see if we can help our young lord. Cause's I know I have been thinking about this issue for too long now.

  • 1
    $\begingroup$ Reliable and sturdy enough... for what? What situation are you envisioning needing this in? $\endgroup$
    – Cadence
    Jan 9, 2023 at 5:27
  • 1
    $\begingroup$ Isn’t the mechanism of any telescoping thing by definition going to be one part sliding out from another. Can you try to be clearer about what exactly youre asking about? $\endgroup$
    – sphennings
    Jan 9, 2023 at 5:27
  • 3
    $\begingroup$ I don't think that being in the barrel longer means the bullet accelerates longer (unless it's a rail gun). Pretty much all acceleration occurs in the first inch or two as the explosive expands, but it doesn't expand to fill the whole length of the barrel. Long barrels are mainly useful for increasing accuracy via rifling, which I assume actually slows the bullet down a little through losses to friction. $\endgroup$
    – Tom
    Jan 9, 2023 at 6:28
  • $\begingroup$ Sounds more for show than performance, but if that's what the ladies like... $\endgroup$ Jan 9, 2023 at 7:49
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    $\begingroup$ My 2 cents: don't explain this at all. Just have telescoping barrels. They have developed "a technique" that makes it work and unless you really need to describe the manufacturing details for some reason to advance the story, just never explain it to that level of detail at all. $\endgroup$
    – JamieB
    Jan 9, 2023 at 14:15

5 Answers 5


The big issue is your internal diameter of your telescoped Barrel...

But there's two possibilities that spring to mind:

1: Sub-calibre, Sabot'd projectiles - essentially, as the projectile flies down the telescoped barrel, the Sabot is 'caught' on each telescoping segment, until the final projectile leaves the barrel. There would need to be a mechanism to clear the sabot components out after firing - most likely collapsing the barrel and re-extending it, kinda like how some Tanks do the 'tank salute' after firing.

2: In much the same vein, Squeeze bore projectiles - if you could make the inside diameter contiguous with each segment, you could use a squeeze bore type projectile - IIRC, they were used in WW2 to increase muzzle velocity for some tanks (with reasonable effect)

However... if Accuracy isn't your primary concern....

Shotgun. Firing Shot, instead of a solid projectile means that it's much less sensitive to changes in the barrel - so you could get away with any of the above technical issues, it just means that there's no actual benefit (such as increased range/accuracy/penetration power etc.).

  • 1
    $\begingroup$ You could make a shotgun choke as a barrel extension, and they're not merely decorative. $\endgroup$ Jan 9, 2023 at 9:14

Not quite "telescoping" in the usual sense, but...

First part of the barrel (including chamber) is fixed. Around it is a closely-fitting sleeve, and then around that are the extension of the barrel in 3-4 longitudinal segments (think barrel staves). Those segments extend forward and move inward, interconnecting with the end of the main barrel and with one another, and then the sleeve slides forward to surround the joint and the segmented barrel, providing additional hoop strength to it.

The machining tolerances would be tight and it'd probably blow up in your hand given half a mind to, but at least on paper it sounds like it'd work. 😅

(note: Not recommended for use with +P+ rounds. For novelty use only. Use at your own risk.)

  • $\begingroup$ Good thing the young lord is fueled by advanced engineering, nano-engineered composites and sheer desperation! $\endgroup$
    – Seraphim
    Jan 10, 2023 at 7:36
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    $\begingroup$ Not to mention 'shear stupidity' - the uncontrollable urge to separate life from body. $\endgroup$ Jan 10, 2023 at 13:22

German ww2 tech has the answer for you:

Research into the Krummlauf has basically stated as long as the barrel extension is firmly attached and centered, a same diameter barrel extension is good enough.

The rifling doesn't even have to be matched perfectly either. Source: Forgotten Weapons (video)

Granted this is Curved barrel extensions. I'd wager that more detail oriented research would be need to be done but generally bullet would be effectively travelling down a longer barrel and stands a small chance of fragmenting in the barrel like was in the Krummlauf if the extension was not curved.

This will not be telescoping though, but can be a folding or bolt on extension.

As for a true telescoping barrel, I'd probably suggest the barrel being split into two sections lengthwise with flanges that'd allow for a clamp to slide over along with a spring loaded barrel sleeve that'd slide the clamps in place. in a stowed position, the barrel halves would be sitting beside the base length, and when a latch is released, the barrel sleeve first drags the barrel sections into place and then slides the reinforcing clamps all in one motion. Since you said your cuttlefish had very advanced manufacturing techniques, I'll not worry about tolerances or complexity.

If we're talking about a gun that isn't strictly gunpowder based and is a gauss gun or even a railgun, then they could easily have collapsing rails that the bullet would ride on and the coils would simply compress back into one place whereas there was space between them when fully extended for firing.


Paper yoyo.

enter image description here


Your extendible barrel is a concentrically coiled flat strip.

At full extent, the interior overlap between adjacent sections of strip serves as the rifling of the barrel.

  • $\begingroup$ I can foresee problems in maintaining a consistent diameter. $\endgroup$ Jan 10, 2023 at 0:18
  • 2
    $\begingroup$ Maybe make it out of a material that has a memory. $\endgroup$ Jan 10, 2023 at 0:19
  • $\begingroup$ /has a memory" - neat idea. It should be possible to achieve an even diameter but a gradually growing diameter seems like the natural. If the wrap became tighter on extending that could even it out. That would be testable with a toy piece of nitinol wire. Now where did that nitinol wire get off to... $\endgroup$
    – Willk
    Jan 10, 2023 at 1:01
  • $\begingroup$ It is in the left hand drawer, third from the top, in the cabinet in the other room, in the other house, in the other city, in the other country. $\endgroup$ Jan 10, 2023 at 3:07
  • $\begingroup$ Thanks! Be right back. $\endgroup$
    – Willk
    Jan 10, 2023 at 4:16

What do you need for a barrel to be effective for gas projectiles? You need precise sizing and rifling.

Putting aside the issue of materials, the problem I see is automating the process. It seems like you want the barrel to automatically slide out. If your character carried multiple segments of barrels and clipped/screwed them together to suit the particular situation, you'd be fine. You could more or less do the same thing now. There are a number of firearms that can accept different barrels and bolts easily and quick-release barrel systems are not uncommon.

If your barrel was a long, coiled sheet that could stiffen so that it could extend out (like the paper yo-yo above), I do not see how a rifling groove would work. You could extend the barrel, but the rate of coiling would change along the whole barrel, altering the rifling twist rate. Rifling changes a musket in a rifle and is hard to get around.

If your barrel was stored in segments or segmented sleeves that rolled out and clacked together, I don't know what mechanical force would be strong enough to hold them together when the projectile and the gasses pass through, even if you don't worry about the barrel itself withstanding the forces.

I think the best way to go is to change the game. Gyroscopic stabilization is not going to work for you. You likely want a magnetic impeller or a railgun.

If you go with a linear accelerator you want to have projectiles that are thin and light so that you can accelerate them to high speed quickly. You don't spin them because they aren't stabilized. They move so fast and their mass is so low their trajectory is basically laser-flat. They're essentially hypersonic needles. In that case, each barrel segment could be an acceleration stage of impeller coil and a gating electromagnet. Adding more length to the coil would always give you more distance to accelerate. While you could just dump more power in to a shorter segment to accelerate to the same speed in a shorter distance, the electronics may be unable to handle switching that much power at once and accelerating that fast might warp the projectile, causing it to impact the barrel going extremely fast (which would be 'bad'). You are relying on linear acceleration and not gas pressure, so you don't need to hold the barrel together all that tightly. There will be resistance, but given you already have a wire coil, you more or less just need a sleeve to hold it.

As for the actual movement of the barrel, I could see largely two solutions. One is the paper yo-yo layout above. If a material sheet had impeller lines built in, then it could slide out and in. The only issue is that the coils at the back of barrel would be tighter than the front, and you want it to be the opposite way around. The other solution is a bullpup design.

Bullpup rifles have their magazine and receiver behind the handgrip, so by the time the barrel runs from your shoulder, along your arm and to your hand, you've already gone through over a foot of barrel before you hit the grip, so the overall length of the firearm is rather short. If your impeller gun had a barrel along the whole length, but only fired through the segment past the handgrip and used the rest of the stock to essentially store the rest of the barrel, it could then be ratcheted out as needed. The barrel couldn't be one solid piece since you'd need a way to get the projectile in, so it could be constructed in segments. It wouldn't be hard to have an internal mechanism hold the front in place, screw in the next segment and roll the front forward.

It would not be quiet, though. Motors generally only get louder over time. It could be manual, like a bolt-action rifle.

While the same setup could in principle work for a gas projectile, the fact that the barrel could unscrew in multiple places would present problems. The stress of the gases and the projectile's rotary motion as it traveled through the barrel would cause all the joints to loosen. It would essentially disassemble itself. Parts can be reverse threaded, tightening themselves each time you fire, but then you need to overcome how tightly they need to be (and then became..) screwed together in order to extend or retract.

I think a bullpup extensible linear impeller is your best bet.

You would need very good power storage and delivery, however you mentioned advanced science and materials are available.


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