I can dig the reason for a powder charge consisting of grains of a certain size. It controls the rate of burn which is critical for producing gas to push the bullet/shell out. Has there not been a solid propellant charge in lieu of a powder charge? I thought the right burn rate in the former can be engineered by using the right explosive and an inert filler to further control the rate. The advantage I see is easier manufacture and “bullet assembly.” It will also allow developers a choice of “composite charges” wherein a fast-burning core will burn the slower burning bulk in a center-to-edge fashion, instead of end-to-end. This end-to-end sequence is what’s seen at present. It’s rather wasteful as some un-burnt powder actually exit the muzzle.
Have you looked at some of the literature on caseless ammunition? Both the Heckler & Koch G11caseless rifle concept and theLSAT rifle concept utilize caseless ammunition, where the propellant is a formed block. Metal Storm also utilized an electrically fired, solid block of propellant for its concept.
I wonder if liquid propellants might be the way to go in the future?
It never occurred me. Thanks. They’ve also considered gasoline guns (vapor, not liquid in that case) and even liquid hydrogen for navy ships. Liquids are messy. In the case of liq H extracted from seawater, you also produce chlorine gas which is bad news,
I’m not seeing the advantage in your proposal. un-burnt powder blowing out of the muzzle has more to do with barrel length than anything else.
That may be correct but the advantage(s) of complete burn in the chamber is proven. Elmer Kieth and a buddy brazed a narrow steel tube inside a magnum cartridge so that the primer flash will ignite the powder at the middle, not the end. In doing that they discovered that for a given amount of charge, you get a tremendous reduction in chamber pressure, which allows you to gob in more powder than usually allowed.
Shouldn’t get any chlorine from electrolysis of seawater. Breaking down table salt requires some serious heat.
But hydrogen has a lot of other problems, it’s really not a friendly substance.
Liquid propellants–in particular hypergolic binary propellants–have been considered for larger guns like artillery, but are probably not practical for small arms without some radical re-engineering, and since most hypergolics are also toxic there are significant processing and contamination issues compared to dry powder and flake propellants.
As for cast or pressed solids, one of the most significant issues I see is the action time; that is, only the exposed grain area can burn; with a powder there is a lot of surface area and so it burns quickly in a short action time. With a solid cast grain the surface area is small compared to the volume unless the web thickness is very thin, and the action time to consume the entire grain is likely much longer than the interval required for the bullet to transit and exit the barrel. This is why cast or pressed grains are used in rocket motors, when you want a long controlled burn, and loose powder is used in guns, where you want a quick pressure spike.
Stranger
Electrolysis of “seawater” is the primary way chlorine gas is produced:
I’d ask you for a citation, but you won’t be able to provide one.
Hell, I discovered that electrolysis of saline produces chlorine myself, at the age of 9, when I tried to speed up a home water electrolysis experiment by adding salt to water to increase its conductivity. I got chlorine in the positive terminal test tube instead of oxygen, but I recognized what was happening because oxygen isn’t green. 
The joys of a childhood with science toys. I had a lot of fun with that chemistry set.
That requires special equipment to keep the chlorine separate.
But even if the chlorine comes out easily- boil the seawater, then hydrolyse the steam. Pure hydrogen and oxygen that way.
But hydrogen still is problematic as an energy storage medium.
Long story short, yes you could replace the powder in a cartridge with a slug of solid propellant, but the advantages would be minimal compared to the extra cost. The biggest efficiency of powder is that it can be prepared in bulk and measured out as needed to different cartridges, as opposed to custom sizing solid propellant for different casings.
The Gyrojet pistol and rifle fired solid-fuel rockets, but I don’t think that’s what you’re going for, here.