I realize this is an old post-- but gunpowder and firearms design is a topic close to my heart so I thought I would add some ideas.
Certainly, a magnetic or other energy source approach to a firearm is within the reach of current technology. This prompts me to toss out some suggestions for the specifications such an alternate approach might want to meet. At a very early point in history firearms were revolutionary-- but since then the development of firearms has been evolutionary-- and there are lots of unique aspects that a new design might want to consider.
The energy source: Certainly, any energy source that cam impart sufficient velocity and momentum will do. Gunpowder is particularly well suited to this task. It is easily packaged in the cartridge which means no external power source has to be maintained. It lasts very long times without degradation (100year old ammunition usually works just fine). This gives it a degree of superiority over liquid fuels and external power sources.
The projectile: accuracy over a great distance usually requires stable flight and a minimum of velocity loss with distance. This prompts us to want low drag and high mass. Low drag to minimize energy losses and high mass to provide higher energy and momentum. Lead projectiles, although originally convenient because one could make their own musket balls over a fire, work particularly well (regardless of whether or not they are covered by a metal jacket) because lead is a very dense material-- allowing lots of mass in a small package with a small drag cross-section. Your design needs to think about the projectile, how far you want it to go and how much energy you want it to retain over that distance.
The launching mechanism: Unless your projectile can be controlled during flight, you need to give it not only linear momentum but also angular momentum about the axis of the direction of travel (spin). This is another reason lead has worked out so well-- it engraves to the rifling, which imparts spin, yet it (and copper jackets) are soft enough that they cause minimal wear to the barrel. And-- a rifled barrel is really a very simple means of imparting spin to a projectile. your design needs to consider how you will obtain stability in flight and how to do so in a way that will be reliable for extended use.
Energy impulse: The time duration and peak of the energy impulse has a lot to do with the requisite strength of the launching mechanism. This actually makes something like a rail gun look pretty good, because the acceleration force can me modulated and maintained right up until the projectile leaves the mechanism. This is, in fact, how gunpowder systems work. There are currently about 150 different formulations of gun powder commercially available that have purposely designed burning rates ranging from very fast (essentially an explosion) to very slow (more like solid fueled rockets). This permits each cartridge to be designed such that the gunpowder burn will last until the projectile leaves the barrel and also allows the peak pressure of the burn to be matched to the capabilities of the firearm design.
Flexibility and ease of use: This relates to the hobby of reloading. Many people (myself included) reload our own ammunition because it allows us to customize the ballistics of our firearm for whatever purpose or interest we wish. We can tailor our charges for any velocity from barely enough to leave the barrel all the way to supersonic flight (within the pressure capability of the firearm). We can vary the composition, mass and shape of our projectile to suit our interests. Some of us (particularly old engineers) can waste thousands of rounds of ammunition doing nothing be experiment with the variables. The equipment needed is simple and inexpensive. This is why reloading is such a popular hobby even though factory made ammunition is readily available-- the factory loads are very standardized and not customized to a particular firearm and its mechanical tolerances. So-- if you want to design a good replacement for gunpowder firearms, think about whether or not you design will be within the reach of people to tinker and play with.
(for those wondering what I do with all this-- I plan to someday have a perfect set of custom loads for each of my 5 rifles-- just for target shooting, since I am not a hunter. So far I've only built and fired about 18,000 rounds, but I still have so very many experiments I still want to try).