It has a maximum capacity of 50 charges. The ammo recharges, giving you unlimited shots. In the first firing mode it fires a weak shot repeatedly. In the second firing mode you can charge the weapon for a few seconds and fire one powerful shot. This weapon can be upgraded, so it remains useful throughout the game.
Upgrades increase power and cell capacity.
It also has a cool little animation as it upgrades. It will kill anything. You find the AutoMag near the start of the second level and is a pretty basic weapon. In the primary mode it fires very accurate shots, but it's slow. This gives a faster, but less acurate firing mode.