Michael Bane provides a link to a study whose conclusion was simple: to stave off rape and/or sexual assault, fight back. In one case outlined by the study, a man raping a woman forced a kiss upon his victim and she bit his tongue. Yes I know, sounds gross, but it was effective.

From the article:

Forceful physical resistance is an extremely successful strategy. The completed rape dropped to 14% when the rapist’s attempt was met with violent physical force. Striking was more successful than pushing or wrestling. Physical resistance also appeared to be more effective when assault occurred outdoor[5].

Women who used knives or guns in self-defence were raped less than 1% of the time. Defensive use of edged or projectile weapons reduced the rate of injury to statistical insignificance[7].

Emphasis added.

Whenever you undertake a task you always do well to have the best tools for the job. And while a weapon, such as a gun, is a mighty effective tool in such situations, the most important tool is to have the mindset to fight.