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Ironically, it would be thought that with modern technology, the "next step" for fire weapons would mean to produce direct energy weapons such as Rail Guns, Lasers or Microwaves, however, because of new techniques in mass producing composite materials, it allowed the armies to use air rifles.

They allow a greater rate of fire (5000 rpm [assault rifles achieve 600 to 900]), far more silent, incredibly precise, mechanically simpler and allows for projectile modularity (since you don't need to exactly change the size of the barrel to achieve the same pressure, unlike powder).

Supposedly.

However, I'm still struggling to find a rough estimation of how big a pressurised cylinder would be to let an Air Rifle to be as deadly and have the same range (or higher) as normal Assault Rifles.

Supposing the rifle shoots the same type of projectile as a M16 rifle, achieving the same range, same amount of ammo (let's assume 6 magazines with 30 rounds [323280 joules in total]) with any highly compressible gas (such as CO2), how big this cylinder would need to be?

Also assuming that the cylinder is made of metal, so it is easier to find real world comparisons. The Composite cylinder doens't decrease in size, just in weight (I think), so, if this cylinder is the size of a soldier, then it is just too bulcky.

The only information I found was this link of Wikipedia where it shows the amount of joules that some calibers releases: https://military.wikia.org/wiki/Muzzle_energy

The size of a M16 caliber is 5.56 mm, which, according to the link above, means the cartridge releases 1796 joules.

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    $\begingroup$ There's also - the quicker the rate of fire, the quicker the cylinder cools and drops the pressure - heat-sinks to re-heat the cylinders might need to get a bit bulky. Why not consider Hydrogen instead - then you can have a built-in heater for the stored gas. $\endgroup$
    – Escaped dental patient.
    Commented Oct 20, 2021 at 0:45
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    $\begingroup$ If you have a compressed tanks of both hydrogen AND oxygen, and then inject them in a chamber, you end up CREATING your pressure. $\endgroup$
    – Justin Thyme the Second
    Commented Oct 20, 2021 at 1:31
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    $\begingroup$ A casual search shows that the M16 has a 52,000 psi chamber pressure which, to put it mildly, is quite a bit and you want a container that holds enough total gas to fire 30 rounds at that pressure, meaning that it has to start at an even higher pressure so that the last round fired still has 52,000 psi in the tank to work with. You can find information about the necessary calculations and heuristics for estimation by looking up "hoop stress for gun barrels" but the numbers aren't pretty. $\endgroup$
    – GrumpyYoungMan
    Commented Oct 20, 2021 at 2:05
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    $\begingroup$ Re the alleged advantages: "silent" - no, supersonic projectiles will always result in a "crack" although possibly a reduced "thump". (The actual advantage is that at night there is no muzzle flash for the enemy to aim at.) As for the rate of fire - except for particular usages such as a 3 round burst setting, greater rate of fire just means less controllable recoil and expending ammunition faster. Not sure how it is mechanically simpler, either, given that it now has to separately feed projectiles and very high pressure gas. $\endgroup$
    – KerrAvon2055
    Commented Oct 20, 2021 at 3:27
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    $\begingroup$ I think there is some misunderstanding of how guns actually work here. @KerrAvon2055 mentioned how bullets breaking the sound barrier is the actual cause of the sound, rather than the explosion of the propellant. On that note as well, explosive propellant guns ARE gas powered, the gas is just produced by the ignition. That's actually how most automatic weapons work, some of the gas is diverted to a piston that reloads the next round. What you seem to be describing is more along the lines of a literal BB gun that somehow has the force of an actual rifle. 1/2 $\endgroup$
    – TitaniumTurtle
    Commented Oct 20, 2021 at 9:23

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'As deadly': doable. for one shot. The 'Texan' is marketed as 'the worlds most powerful production airgun' and is supposed to reach about 800J of muzzle energy. For comparison, a NATO 7,62×51 is listed by Wikipedia as having around 3000J of energy (NATO 5,56×45: 1500J), so while the airgun is somewhat lacking, it is already in the same ballpark. Airguns for research purposes often use hydrogen or helium as propellant gases, because their speed of sound is higher, enabling higher velocities for the bullet - also, higher temperature of the gas is good, which they achieve... with a real gun

But lets assume we have a working gun, the burst disk from the research guns has been replaced by a special valve (or has it? maybe your gun just has a big supply of them) and the gas is somehow heated (electric arc? would fit the theme: an air-electric rifle sounds awesome) and we now can get a projectile to competitive muzzle energies - how often will we be able to shoot? Ideally, the bullet gets pummeled by compressed gas all the way to the muzzle, meaning that the entire barrel needs to be filled with compressed gas. let's say that is a 1.28cm diameter barrel, and we assume a length of 50cm, that will be 50ml.

These are 50ml at high pressure, near the pressure of your storage, so let's just pretend that is the kind of storage volume you would need. That makes a 1l vessel capable of 20 shots, which is not realistic of course, because the vessel looses pressure every shot... The Airguns of yore (mid 1800s gun) had that same problem, loosing power with every consecutive shot. Todays composites allow the airtanks in airsoft to be at 30MPa while being quite portable, steel tanks are not really bigger, but just very unwieldy due to their weight.

Energy density is not very good for compressed gases - additionally, the way you get the energy out of there matters a lot, and 'violently pushing small object down a barrel as fast as possible' does not even begin to be an efficient way of getting your energy out there. Coaxing the energy out in a slow, multi-stage process, will yield 300 kJ from a 5l bottle a 20MPa, already discarding another 230kJ that are lost, or stay in the bottle which left filled to 1MPa... (300kJ was also about the energy you cited for your soldiers loadout) - 1l Paintball tanks at 30MPa will give you about 1000 shots at about 10J, so 10kJ are harvested, while the above example shows that for the same volume at only 20MPa you could get 60kJ

But maybe you should own and compensate the drawbacks of the airrifle and not make it about pure muzzle velocity (and also not about muzzle energy - kinetic energy scales with mass, but it also scales with velocity squared, so there is no way to keep up an energy arms race if you cannot score velocitywise) while playing to its strengths:

  • Keep the gas-supply topped up via chemical reactions: just like cartridges, but slightly slower burning and thus completely silent
  • keep muzzle velocity below the speed of sound for gun-noiselessness and reduced requirements regarding pressures and gas-types
  • keep the barrel very thin to avoid losses
  • gently shoot stuff that does not like being explosively propelled (seeker-bullets? bullets with little razor wings that swing out and help with range? bullets with some kind of tracker (for the big guns to target?) ?)
  • rev up that rpm (no barrel overheating, if anything they might get frosty; no need to get rid of cartridges)
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    $\begingroup$ Re: gently shooting stuff that doesn't handle explosive propulsion, this was actually used in the "dynamite gun cruisers", e.g. USS Vesuvius. Their dynamite shells were too shock-sensitive to use regular propellant, but they certainly had a kick when they landed! $\endgroup$
    – Cadence
    Commented Oct 23, 2021 at 0:25

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