My scifi settings armory is filling up with many interesting weapons. However, while energy weapons, Gyrojets, Tangler guns and the like outperform slugthrowers in special applications, concentional slugthrowers still reighn supreme as all-round guns. The wide array of special ammunitions make them even more versatile and useful. Sure, a Gyrojets fire rate and heat management in vacuum or an IR-laser-snipers stealthyness can't be achieved, but carrying 15 different kinds of special ammunitions allows the slugthrowers to keep up by being versitile.
I do want to mix up slugthrower design a bit to drive home that this is the future. While handheld rail- and gauss guns exist, they are more of a novelty like plasma guns, than actually useful weapons. What caught my attention however are combustion light-gas guns. These guns use oxygen-hydrogen or hydrocarbon mixtures to propell their projectiles. The wiki article says that muzzle velocities of 3 km/s are achivable, but that they have a poor accuracy at artillery ranges. This is bad for artillery applications, but doesn't really limit the use of these a vehicle mounted or handheld guns.
Advantages and Disadvantages
the fuel, be it hydrogen, methane, ethanol or a higher order hydrocarbon, is readily available and cheap; especially spacecraft will have a pretty much infinite supply of "gun gas"
the fuel is harder to handle; while hydrogen gas has an amazing gravimetric fuel density, the volumetric fuel density is atrocious; carbon containing fuel alternatives metigate the issue a bit, but are still hard to handle; however advanced material science could solve this, a fuel cartouche made of graphene will make handling high pressure hydrogen much more pleasant
increased muzzle velocity allows one to either go for more damage or to reduce projectile mass while keeping the damage on a similar level, this in turn would increase magazine capacity significantly
Are combustion light-gas guns capable of competing with conventional solid fuel ones, given the technology matures significantly? What would be other advantages and disadvantages? If they can't compete, what would have to happen so they can?
Edit1: This is a conventional gun, except that the gunpowder has been replaced with explosive gas. It shoots normal bullets, not Gyrojet Rounds.