Combustion Light Gas Gun (CLG Gun):

- Instead of solid propellant they use a mixture of 'light' Hydrogen and Oxygen gases similar to what Space Rockets do.

- "CLG guns utilize the explosive force of low molecular-weight combustible gases, such as hydrogen mixed with oxygen, as propellant. When the gases are ignited, they burn,
expand and propel the projectile out of the barrel with higher efficiency relative to solid propellant and have achieved higher muzzle velocities in experiments.

Combustion light-gas gun technology is one of the areas being explored in an attempt to achieve higher velocities from artillery to gain greater range." [WPD]

-In order to achieve the insane muzzle velocities CLG guns produce, the mixture of gasses are often highly pressurized before ignition.

Pros:
-Lethality [These are above-water calculations]:
-->45 mm CLG gun demonstrator - Muzzle Velocity: 7.2 km/s (Mach 21.7 Insanely high, explosive levels of Kinetic Energy!)
-->Railguns: 4 km/s
-->Conventional Max: 1.8 km/s
-Greatly Enhanced Range
-->CLG Guns: 200+ km
-->Conventional Artillery: 30 km
-->Tank Cannons: 4 km
-The simple and common Oxygen and Hydrogen propellant can be manufactured on-site using Water Electrolysis----Submarines use this to split water into Oxygen and Hydrogen using electricity.

Cons:
--->Due to the very, very low density of natural Hydrogen (0.08988 g/L) and Oxygen (1.429 g/L) gas, to have anywhere close to a combat feasible amount of
ready-to-use ammo while not taking up too much space the Oxygen and Hydrogen must be kept cryogenically frozen at −259.16 °C (−434.49 °F!)----this ups the
density greatly and thus lowering space requirements to acceptable levels:

Gaseous Hydrogen --> Liquid Hydrogen - 788 times more dense
-->Must be kept at −252.87 °C or −423.17 °F [20.28 K]

Gaseous Hydrogen --> Solid Hydrogen - 956 times more dense.
-->Must be kept at −259.14 °C; −434.45 °F [14.01 K]

Pure Oxygen Gas --> Liquid - 798 times more dense.
-->Must be kept at −182.96 °C; −297.33 °F [90.19 K]

Pure Oxygen Gas --> Solid - 970 times more dense.
-->Must be kept at −218.79 °C, −361.82 °F [54.36 K]

Cons Continued:
--->Both pure Oxygen and Hydrogen are extremely flammable.
--->If the cooling fails and the emergency pressure relief valves are stuck shut (lack of maintenance) the liquid propellants would quickly turn to gas and expand rapidly causing a violent pressure explosion which could itself ignite the highly flammable pure gasses.
--->If the tank is compromised in battle the relief valve will only prevent a pressure explosion as internal leaks will still be a massive problem.
-->If one of the tanks has a leak even the slightest spark could ignite the entire tank and the escaped gas.
-->Hydrogen is especially good at 'finding ways out.'
-->If the escaped pure oxygen were to come into contact with machinery oil or grease (engine room, pump room, fabricator room), it would react to
form a waxy substance that when simply stepped on would explode violently, the wax will also explode when exposed to even the smallest spark.
-->Almost any material will burn vigorously in oxygen including rubber, silicone and even metals.
-->Pure Oxygen-fueled fires are ferocious and more difficult to put out, as they burn hotter than other chemical fires (1500+ °C or 2700+ °F).
-->If both tanks are leaking for some reason (degradation, battle damage) the mixture of hydrogen and oxygen when ignited would be even more explosive in even smaller amounts
(After all that is the same mixture the CLG gun uses [although in higher pressures and thus concentrations] to propel projectiles to 'mega-hypersonic' speed in air).

--->All these safety issues mean the tanks themselves would need to be manufactured to a very high standard (expensive).
--->The tank set-up can take up a lot of space depending on te size of tank.

--->The shell, the fuzes, explosive fill, and guidance systems all must be "hardened" against the significant acceleration and friction loads to survive and function properly (applies to both set-ups, the other mentioned below---impacts logistics expenses considerably)

Alternative single-use cartridge set-up:
--->The single-use, pre-packed cartridge set-up where the propellant is behind the perpetrator/warhead would save space (likely using the railgun racks),
but would likely be extremely expensive rounds to buy due to the complexity of containing a mixture of highly pressurized, highly flammable gases.
-->These single-use rounds would only be exempt from the extreme cooling requirement and maintenance for all the other dangers with these gases still apply.
-->These shells could possibly have a health-bar that degrades under certain conditions and ofc when taking direct damage; when the health-bar reaches certain
thresholds small leaks start forming---75% maybe, below 50% larger leaks form, below 25% all the gases escape within 5 seconds, if the shell hits 0% before all the gas escapes
there is a pressure explosion---size depending on how much gas was left in the shell when it hit 0% health.

For the shell's visual design:
It would probably use slightly heavier, larger, hardened, specialized, and advanced looking shells over the rail-gun [to survive those extreme accelerations].

More Info:
https://www.youtube.com/watch?v=Axmmzf8Au0A
Boom:
https://www.youtube.com/watch?v=zFyqilT0ld0

Overall:
CLG Gun is basically a better Railgun in many aspects:
--Power consumption
--Lethality
-->MUCH better penetration, damage, range, velocity.
-->Decently better maximum potential reload rate
-->Slightly better turret traverse;
although there isn't any rail-accelerator weights holding it down, the barrel and shell chamber still has to be very bulky and heavy due to the insanely high chamber pressures the CLG gun outputs (both set-ups).

However, not better in these aspects:
--Cost for installing (probably the most expensive weapon to install yet)
--Cost for ammo (mainly the cartridge set-up)
--Maintenance (mainly the tank set-up)
--SAFETY----the potential disasters can be catastrophic (as detailed above)!

Trade-off:
Tank Setup:
You save a lot of money in the long run buying the specialized shells,
but you trade that for additional maintenance and potential safety risks.

Shell/Cartridge Set-up:
You save some initial fees on not buying the tank set-up as well as
a slightly lower amount of potential safety risks, but in the long run
the ammo expenses will add up very quickly.

Thanks for reading, if the Railgun that has been in experimental development for some future navies is already in game; why not the CLG gun which, recently, has also been in experimental development.

Let me know what you think. If you need any more info please let me know.