8d8 damage in a straight line, penetrating enemies (so you can hit a whole line of them) and punching single-block holes in walls...
1.5 second firing time, +6 accuracy, I think 2 second reloading time (not sure on that one).
holds 40 cells as ammunition, uses up 5 cells per shot.
With nanopack it regenerates 1 cell per second, so by the time youve fired off all 8 rounds, youve regenerated enough ammo to squeeze out another 1 or 2.
Just as a rough guide, 8d8 damage tends to kill most monsters in a single shot. Sometimes pain elementals, and usually barons, archviles, and mancubi take a second shot. Revenants, hellknights, cacodemons, demons, and former humans all go down to a single shot.
As an example of why its so good - I cleared the mortuary just by walking up and down on the far right of the level, firing to the left, until all the monsters were dead. Didnt even get to see a monster more than once or twice. On a typical level you can just wait a few hundred turns, then start firing towards the center of the level and watch your experience go up. Basically any level can be surgically dismantled and its monsters slain without putting you in danger, and without any items being taken down in a blast.