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The idea would be a typical rifle cartridge with a case and propellant but with a gas section placed between the case and propellant and bullet with a soft metal disc that would be driven by the propellant compressing the gas similar to a mini-light gas gun or Voitenko compressor where the higher speed of sound in the light gas working fluid increases the maximum possible speed. Would this allow for higher velocities or more effectively accelerate the bullet.

Even meager increase in velocity is acceptable as the context is breaking the soft velocity cap achievable limited by the expansion rate of regular propellants in cartridges for increased velocity in lower tech sci-fi high-velocity AP rounds.

Wikipedia diagrams of light-gas guns and Voitenko compressor are relevant but I didn’t include them as to not make the question confusing.

Edit: added excerpt from light-gas gun Wikipedia page for context

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  • $\begingroup$ The usual limiting factors in bullet velocity are the barrel strength/length and the type and quantity of explosive used. We've things much better than nitrocellulose, but the barrels get too heavy to contain them without themselves exploding. You've not specified any propellant - the amount of energy imparted to the projectile depends on energy released by that. It's tough to see what a whiff of hydrogen will do to change that. $\endgroup$
    – Escaped dental patient.
    May 28 at 7:41
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    $\begingroup$ In my understanding there is a maximum muzzle velocity that can be achieved regardless of quantity of propellent limited by the expansion rate of the propellent which is in turn limited by the speed of sound which is why in a light-gas gun a regular shotgun shell drives a plunger which compresses hydrogen which has a higher speed of sound allowing for higher muzzle velocity. In a Voitenko compressor the hydrogen area accelerates the shock waves due to this. I apologize if I'm incorrect I don’t understand physics very well. I am only trying to explain more clearly the original question. $\endgroup$
    – J. Y.
    May 28 at 7:57
  • $\begingroup$ I need to accept that there's more going on here than I yet understand. $\endgroup$
    – Escaped dental patient.
    May 28 at 9:39
  • $\begingroup$ Thank you for your time regardless $\endgroup$
    – J. Y.
    May 28 at 10:01
  • $\begingroup$ Hi JY, it's common practice to not select an answer for at least a few days after the question has been asked, as others are less likely to provide answers of their own if they see the green tick $\endgroup$
    – M S
    May 28 at 14:42

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No, this would not work.

The disc between the propellant and light gas can't be driven faster than a bullet would by the same propellant, and it can't in turn compress the gas any faster than it is itself traveling.

In order to accelerate the gas, you need to compress it further by forcing it into a funnel, as described by Wikipedia:

One particular light-gas gun used by NASA uses a modified 40mm cannon for power. The cannon uses gunpowder to propel a plastic (usually HDPE) piston down the cannon barrel, which is filled with high-pressure hydrogen gas. At the end of the cannon barrel is a conical section, leading down to the 5-mm barrel that fires the projectile. In this conical section is a stainless steel disk, approximately 2 mm thick, with an "x" pattern scored into the surface in the middle. When the hydrogen develops sufficient pressure to burst the scored section of the disk, the hydrogen flows through the hole and accelerates the projectile to a velocity of 6 km/s (22,000 km/h) in a distance of about a meter.

In order for this to work, your rifle would need such a piston arrangement, a mechanism for replacing burst disks, and a substantial supply of hydrogen - more than would be able to fit in a conventional cartridge. It would need separate feed mechanisms to supply propellant at the breech and bullets midway down the barrel. It would be a huge amount of added work, added complexity, and added weight. This wouldn't be a magazine of special ammo you could issue to an ordinary rifleman, it would be a whole heavy weapon in its own right - and frankly it's hard to see what advantages it has over a conventional anti-materiel rifle at that point.

Your question talks about maximizing muzzle velocity, but muzzle velocity isn't the main factor you're concerned with in the anti-armor role. (In air-defense and especially missile-defense contexts, it can be more important because your time to intercept is so short, every fraction of a second counts. But a tank will generally still be there if your bullet takes a split second longer to reach it.) What matters is kinetic energy, which is a function of velocity and mass. Chasing expensive gains in muzzle velocity may not get the same bang for your buck as simply getting a bigger gun.

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    $\begingroup$ I did realise shortly after posting that I should have necked down the cartridge in the diagram so that the gas reservoir is wider than the projectile and not equal to it as then it could not have the effect. Point taken about the amount of gas that would be needed that wouldn’t fit in a conventional cartridge and kinetic energy. $\endgroup$
    – J. Y.
    May 28 at 11:08
  • $\begingroup$ @J.Y. Well, the cartridge could contain very compressed hydrogen so it can pack a lot into a small space. $\endgroup$
    – causative
    May 29 at 1:28
  • $\begingroup$ I looked at 2-stage gas guns to see whether you could fire a bullet from Mars to Earth. This was for returning samples rather than anything H.G.Wells (The answer was probably not, though I could not absolutely rule it out). Plasma guns or rail guns look a better bet, and that is not saying much. $\endgroup$
    – Richard Kirk
    May 29 at 8:41
  • $\begingroup$ @RichardKirk Interesting to know. I would've thought they would be more useful in space where squeezing the absolute highest impulse out of every bit of mass is more critical. $\endgroup$
    – Cadence
    May 29 at 8:45
  • $\begingroup$ That is indeed an interesting idea regardless of it's feasibility $\endgroup$
    – J. Y.
    May 29 at 9:55
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Pneumatic Nail GUN

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The principle is sound. Use compressed gas to shoot a projectile. For proof of concept see the pneumatic nail gun. They use compressed air to drive a nail. The idea is to put the "barrel" in contact with the thing to be nailed. Otherwise the nail gets launched like a bullet.

The only difference is the propellant is not contained in the bullet. Putting propellant in the bullet is a bad idea as it limits the amount you can use to however much can be safely contained inside the bullet. Put the propellant BEHIND the bullet instead. Now there is no excess weight containing the propellant and you can shoot hard as you want.

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    $\begingroup$ Given that the highest "muzzle velocity" nail guns give the nail a speed less than half that of a typical modern assault rifle, the fact that nail guns use compressed air does not prove that the same principle can be used to achieve superior muzzle velocity to conventional propellants. (The speed they achieve is perfectly adequate for its purpose - you don't want a nail blown through a house, you want it to hold pieces of wood together - but that purpose is not what the OP is looking for.) $\endgroup$
    – KerrAvon2055
    May 29 at 3:16
  • $\begingroup$ It’s partly my fault he misunderstood the question after editing in the excerpt there we no further misunderstandings $\endgroup$
    – J. Y.
    May 30 at 0:11

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