On Wed, Jan 14, 2015 at 06:47:27PM +0000, David Shaw wrote:
> I make this a KE of 1.302 exa-Joules, equivalent to a touch over
> 311MT of TNT.  Is this right?

Yes.

> And would such a weapon system really be feasible within an
> Earth-standard atmosphere?

With enough magic, sure.  Obviously you need to completely evacuate
the flight path of air.

It's that, or have a truly mindbogglingly powerful shield protecting
the firer from the nuclear explosion that will occur when the
projectile hits air.  A rough calculation gives an effective range of
a few kilometres, in the sense that a majority of particles making up
the former projectile might reach that range, though with lots of
scattering sideways and some backward.

At those speeds, the physical or even atomic composition of the
projectile doesn't mean nearly as much as it does at lower speeds.
The kinetic energy per nucleon is very much greater than the nuclear
binding energy, so when the nuclei encounter each other they spray
particles in all directions.  The initial momentum matters only while
the projectile is still relatively compact (in the first kilometer or
so), but it would quickly expand and hit a lot more air than its own
mass.

The bulk of the effect would be that of a slightly distorted exajoule
nuclear explosion centered probably a couple of kilometres downrange.
There would also be an intense "tail" of explosive power on a line
between the muzzle of the gun and that area, starting roughly on the
order of ten kilotons TNT per metre (if at sea level air density).

- Tim