Krok

The Krok is a bolt cannon that can be purchased from the arms merchant from your home base for 16500 gold after you have purchased the second module.

It is arguably the most powerful bolt cannon in the game at the moment as it surpasses every other bolt cannon in the game in terms of damage, range and projectile speed by a HUGE margin. It does, however, suffer from a very low rate of fire, but this is easily made up for by the fact that it can kill Marianas with just three bolts before they can get in range to return fire.

Since the Krok only consumes one unit of scrap metal per shot just like the other bolt cannons, it's by far the most efficient bolt cannon in the game.

Notes:

 * The Special Bolt Cannon appears to be roughly identical to the Krok.