The bolt multiplier was a feature presented in all games following Ratchet & Clank. It was earned after the game was beaten at least once and the player entered Challenge Mode. After the feature was available, for every enemy Ratchet defeated, the bolts earned were multiplied by a sequence of numbers, starting in 2 and ending in 20. In Ratchet & Clank: Size Matters and Secret Agent Clank; however, the bolt multiplier could only go up to 12. With this upgrade, it would make it easier for Ratchet to obtain the more upgraded forms of weapons or the different R.Y.N.O. weapons. The more enemies Ratchet killed, the more the multiplier would go up, but dying or being hit, even if not taking damage (as while being equipped with the Shield Charger), would drop the multiplier down to 1.
Ratchet & Clank: Going Commando
This is the first game where the Bolt Multiplier was introduced. Every two multiplier levels, the number of enemies Ratchet has to kill in a row increases by one to gain one multiplier.
In terms of math, it comes down to a rounded y=2√x equation, y being the multiplier and x the number of enemies to kill without getting hit.
Ratchet & Clank: Up Your Arsenal
In this game, the method for increasing the Bolt Multiplier was the same as in Going Commando. Dying or being hit, even if no taking damage, as while being equipped with the Shield Charger, would drop the multiplier down to 1. Being hit while controlling a Turret, driving the Turbo Slider or piloting the Hovership would not drop the multiplier down, but enemies killed with them count towards increasing the multiplier.