The Kiloton radium rifle is a unique radium rifle that bears the Explosive legendary affix. It is therefore quite similar in function to the unique submachine gunSpray n' Pray as they both can be fully automatic and deal an additional 15 points of explosive damage per shot. This property, coupled with the 50 points of radiation damage that come standard with all radium rifles, makes the Kiloton radium rifle an exceedingly powerful weapon.
A kiloton is a unit of measurement used to determine the size of powerful explosive events like meteor strikes and devastating industrial accidents, but primarily for the relative yields of nuclear bomb blasts. A single kiloton is equal to 1000 tons of TNT. The weapon's name therefore probably references its combination of explosive and radiation damage.