Revisiting the volcanic
I was at the shop today pondering over the old cartridges offered by this company and I saw the photos of older, gas checked "minie ball" style cast leads sitting by themselves and immediately I thought of the Volcanic, and then just as immediately I thought, "How would I modernize that?"
Ok, basically you use a standard bullet shape, hollowpoint or otherwise, and you add a skirt. Copper, likely. It'll have a little nook in the forward part of the propellant chamber, to hold a primer. Yes, the primer will be ahead of the propellant. Think needle gun here... though there could always be the option of *gasp* electric spark, peizoelectric, laser, or microwave ignition. Ok, the last are going a bit far. But anyways.... primer within the propellant. The "needle" would be made of (we've discussed this before) tungsten, very likely, for density and vaporization/corrosion/spallation resistance.
Now, the interesting thing about holding the propellant within the projectile is the versatility. You could use a standard burn powder in a binder, or you could use a slow burning propellant behind a lavalle nozzel.... aka a rocket. Or likely both, to get the bullet moving nicely supersonic and then have it accellerate beyond that. But, again, ahead of myself.
Advantages? Lighter weight, no cartridge to eject, mechanical simplicity,
Disadvantages? the needle striker, unique ammunition for a while, exposed propellant, the need to have solid propellant (or a cap cover that could be pierced by the striker, but then you've got debris issues), cleaning issues since the chamber turns into a gas trap
It'd work with pretty much any feed mechanism except a revolver, since the entire round leaves... you'd need a fully shielded cylinder front and rear to keep them from falling out. There'd have to be work done on reducing obturation and "stiction", and chamber stretch as well since the "cartridge" has to be free to accelerate forward down the bore.
Further work would require I sit down and make a mechanical drawing and figure out the density of the alloy used, etc. I'm not up to that tonight, so consider this the "lip service" phase of the modern Volcanic.