The correct answer as I found out from other sources is "Saranga" and therfore Krishna is also known as Sarangapani.
As per my knowledge none of the well known ancient books like Mahabharata or Bhagwad mention the name of bow used by Lord Krishna. As a matter of fact I do not know of any incident when Lord Krishna used bow.
The favourite weapon used by Lord Krishna is a spinning disk called chakra. The chakra used by lord Krishna is named Sudarshan Chakra.
In addition to sudarshan chakra another personal equipment carried by Lord Krishna in battle is a shell (called shankha), used for making war like sound by blowing like a horn. The shankha belonging to Lord Krishna is named Panchjanya.
Mahabharata does speak of a famous bow named Gandiva, this belonged to Lord Krishna's close friend Arjuna. Lord Krishna, acting as charioteer of Arjuna, accompanied and advised him all the time during war of Mahabharata.
Lord Krishna is an incarnation of Lord Vishnu, and Sarang is definitely the name of bow held by Lord Vishnu. However I am not sure if Lord Vishnu used this bow as Krishna also.
From the Bhagavata X and XI Krishna: The Beautiful Legend of God (Penguin Classic)
1: "The King said: "Please recountthe heroic of Bhagavan Krishna, the holder of the Sarnga bow, when he killed Bhauma, and also how those women were held captive by Bhauma."
16: "Then, with his sharp and colourfully feathered arrows, Bhagavan Krishna, the elder brother of Gada, reduced Bhauma's army to severed bodies, necks, thighs and arms, and slaughtered the horsesd and elephants in the same way."