I think the limit has pretty much been reached with smokeless powder. Maybe in the future we'll see some kind of rifle where you'll just stick a bullet in there and the propellant will be injected through the breech? Maybe gyro stabilized sighting systems and no more rifling, instead saboted bullets like the projectiles out of the M1 tank gun. Zero lock time ignitions like are coming about with the electric ignition guns. All in all I think the biggest advances are to be had by increasing velocity to 3-4 times current levels by some alternative propellant, lightweight alloy saboted bullets, and smoothbores. Imagine the kinetic energy of a 50 grain tungsten projectile at 12000 fps that has a ballistic coefficient around 3.0. Essentially you'd be shooting a needle with greater energy than a .375 H&H and virtually no drop over the ranges you can see a game animal. Of course, if you hit a deer with that thing you'd probably splatter parts all over the hillside.

I tend to agree with the others, I'm satisfied with the bolt action rifle in it's current incarnation, but it's fun to wonder what might happen.