For the spinal mount you can probably rationalize increasing the damage dice done. Maybe even do a death star thing, 100d1000, destroy target planet with diameter less than 10.
The Death Star's main weapon consisted of 7 spinal mounts collemated into a single beam. Now the T20 rules tell us that all spinal mount weapons do 16 dice of damage, so 7 such weapons would do 96 dice of damage, d20's in the case of a meson beam weapon*. Rounding up to 100d20 damage (for simplicity) this would give an average of 1050 points of damage.
The burst radius for such a weapon would depend on the USP rating, at 10 meters per USP rating. The book values for meson guns go from 10 to 27. If we assume a value of 25 for each gun, this would give a combinded area of effect of 1750 meters.
But compared to the size of a world, this really is not all that much damage, nor is the area significantly large. What matters here is that this damage was done deep inside the planet. A portion of the core of the planet more than 3 km across was nuked. Turned to vapor. What blew Alderan up was not the explosion of the beam
per se, but the effect of 2.24 x 10^10 cubic meters of rock expanding into gaseous form.
*Since the beam was emiting visible light, we should assume that is was in fact a meson beam based weapon rather than a particle accelerator weapon. A PA would only be visible due to the absorbtion and reemission of energy by particles hit on its path, and thus would not be visible while in space. However, a meson beam could emit a certain amount of light all along its path due to quantum uncertainty effects on the decay rate of the meson particles. There should actually be a similar light beam on the far side of the target as well, as some of the meson particles would not decay as soon a they should
