Energy density and efficiency is the big holdup.
Metalstorm keeps trying to promote their product as having potential for handguns and rifles. However, the main advantage to Metalstorm is that multiple barrels can be banked calliope fashion giving it's insanely high rates of fire. This is good for fixed mounts like armor, ships, and aircraft, but not people. A grid of these barrels is useless as an individual weapon.
With the 4-6 barrels you can get in a handgun or rifle, you lose that firepower advantage quickly. And the other supposed advantages such as intelligently choosing multiple projectiles, varying the velocity on the fly based on rangefinders etc. is actually quite limited, because one particular ammunition has to be in each barrel, and since the rounds are stacked roman-candle fashion the "wrong" rounds can be in front, and have to be fired first etc.
Also, reloading necessitates all new barrels with the rounds pre-loaded in them each time. That gets bulky fast.
So I don't think Metalstorm is going anywhere in individual weapons.
Energy weapons just aren't that efficient. The amount of energy storage needed to make a laser or other directed energy weapon that could actually stop, kill, or significantly maim a human target, or game animals is HUGE. You have to factor in heat dissipation, energy losses in transformational stages, and that's just the "rifle". Energy dissipation in the air from dust, smoke, or even the initial smoke from the "shot" hitting the target can and will reduce it's efficiency immensely.
And that's not even addressing the challenges of optics and focusing mechanisims robust enough to survive infantry conditions.
If an energy source capable of delivering so much energy to the beam that it overcame all the losses, that "battery" would revolutionize the rest of the world first! They'd be putting these in cars, houses, laptops that run for a month etc.
I think lasers will be seen more and more in air combat, missile defense, anti aircraft roles, or ground attack, where you have a large nearly guaranteed volume of clear air between you and the target.
It's not so much that weapons that use something other than a bullet propelled by gunpowder aren't possible, it's that it will take quantum leaps in materials science and energy storage to make them practical in portable applications. And those same quantum leaps will revolutionize several industries and products beyond recognition, not just "guns".
I think the things that will make "guns obsolete" on the battlefield will be systems that employ autonomous micro-robotics. You already see this trend starting with UAV's, and the first armed Predator drones have already made their kills. The "modern" battlefield might see things like swarms of intelligent crawling land mines, and kamikaze micro-flyers the size of birds or large insects that seek out human targets autonomously and explode.
Think Sony's AIBO dog-robot, but smaller, several generations more advanced, and with a chunk of C4 in it's belly. Now some factory in China is turning them out by the millions. In terms of asymmetrical warfare, the suicide bomber and the IED have shown us the way. This will just take the fallible human out of the equation.
It would be almost impossible for someone armed with a "gun" no matter how advanced it was, to shoot every member of such a swarm. Plus the swarm will communicate with wi-fi like technologies and will deploy itself as an intelligent mass. Only one of these bots needs to get through to kill the soldier, and the soldier has to shoot every last one of them to survive. Those are very bad odds.
Instead, there will be counter-screens of protective robots, and our own swarms of offensive bomb robots. And surely there will then be anti-robot robots to clear away the protective screen and so on…
That's the future of warfare. If a human is to survive even five minutes in such a battlefield, they'll need powered armor.
When a city has millions of these things crawling into it to attack, a nuke might look "humane" in comparison.
