Well, there HAVE been some ultra-speed loads from conventional firearms. There is a theoretical limit of about 5400 FPS. It maxes out based on the rate of expansion of the gasses that push the bullet. But bullets start flying apart from the spin. Note that the main gun for the Abrams and related tanks, which is VERY fast, is smoothbore- no spin.
We have gotten velocities on the close order of 10k FPS, but that was using explosives to drive the projectiles (the jet from a shaped charge can travel at about 16,000 MILES per hour!) Self forging projectiles, known as Miznay-Schardin effect. http://en.wikipedia.org/wiki/Explosively_formed_penetrator
There was some work being done with very light projectiles made of polycarbonate driven to some insane speeds with a magnetic driver (rail gun) but actual data on that is classified.
Some of the research on the rail guns was done here in Austin at the University of Texas JJ Pickle research facility. They HAVE a reactor and have attainied speeds of over 11,000 fps! A buddy of mine works there in a different area.
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