There have been experimental artillery systems that used binary liquid propellants.
Presumably one of the advantages was that exactly the right amount of propellant could be injected for the desired range and velocity, giving you almost infinite control.
Not an actual firearm, but I own a cordless nailgun that uses butane and a battery as an ignition source. Think of it as a single-stroke IC engine. If the piston that forces the nails into the wood wasn't permanantly attached it would make a dandy projectile.
The main reason you don't see much divergance from the self-contained metallic cartridge, is that it's extremely efficient.
They're waterproof, almost completely inert when left at rest, much safer than say, gasoline, and with a minimum of care, they have a shelf life in excess of 100 years. Smokless powder cartridges are not all that energetic if accidentaly burned either.
Mechanicaly, it's a heat-sink for the gun, the ejected brass takes a considerable amount of heat away from the firearm's "system" when viewed from a thermodynamics standpoint.
In terms of redundancy, metallic cartridges are a very fault-tolerant system. If one fails, just eject it, load the next, and you're back in business. In the thereoretical non-gunpowder firearm, if the fuel-tank, battery, or whatever, fails, the entire gun is out of comission.
It's an extremely hard standard to beat, especially in man-portable arms.
Heckler and Koch, in conjunction with Dynamit Nobel, developed the first truly practical caseless ammunition for the famed G-11 rifle, however, from a chemical standpoint, the propellant was still very similar to double-base nitrocellulose gunpowder used in conventional cartridges.
