Jon sez

Everyone who was guessing as to what that gun would do to our friendly neighborhood goon is now free to compare their guesses to this page.

From the script:

New Abilene Model 91 railgun
Hugo carries this monstrosity, which is a meter long and is designed to be carried by burly solders in powered armor. The grip, trigger guard, and such are all oversized so that someone wearing power-armor gloves can handle the weapon.

The gun's power can be turned up and down with a pair of buttons (with handy arrow icons on them) beside the trigger guard. Above or beside the buttons is a display showing the gun's current power level. Hugo usually keeps the gun at 5%, where it's man-usable. At 100%, the gun kicks like a tank shell.

I got the idea for this from a comment I once read about a crew that had never seen a knee mortar before, and who thought that you were actually supposed to brace it against your knee. Needless to say, the human femur isn't meant to take the launch force of a mortar round.

Mark decided to replace the power-designation buttons and power level display with a nice big knob to make it clearer what Hugo is doing when he turns it up.