2. Liquid propellant guns (search on that term) are well developed for artillary, but I don't know of any light weapons which use this. LPGs are kind of neat in howitzer type applications because (1) A tank of propellant & a rack of projectiles takes less space than cased solid propellant shells, so you can carry more ammo, and (2) you can vary the propellant from shot to shot based on emergent conditions. One neat hack is 'time on target' in which a series of rounds are fired in quick succession, at different elevations and propellant load so they all arrive at the target simultaneously. (the LPG liquid propellant does not need an added oxidizer).

Peter Trei

Indeed. See http://yarchive.net/mil/liquid_propellant.html My ideas were more along the mono-propellent direction using hydrogen peroxide or N-Propyl Nitrate decomposed on a silver/nickel catalyst bed.