Once you've achieved maximum velocity with a case full of powder, the only way to increase the velocity is to use a faster burning powder. With the faster powder you'll get higher chamber pressure, the problem lies with the pressure curve. If you graph the amount of powder used and the pressure it creates, you'll see a linear increase in pressure as the quantity is increased, up to a point. Once pressure hits a certain point it changes the way the powder reacts, and the pressure curve takes off like a rocket. I have no doubt that in the future smokeless powder will be looked upon as a primitive weapon. Some chemist will discover a propellant that'll drive bullets like a laser beam, and new barrel steels will be created to handle the extra pressure. Until then we're stuck with our current limitations.