The actress, who stars as Baroness Anastasia DeCobray in G.I. Joe: The Rise Of Cobra, told London Lite that she was forced to learn how to use firearms for the role.
Miller said: "I can now do martial arts. I can fire weapons and I'm a good little scrapper. I had to get fit for the first time in my life.
"I did six weeks of fight training and initially firing guns was quite a big deal. I found it intimidating, not coming from a gun culture, and I couldn't do it without squinting."
Miller recently said that she hoped to win greater box office credibility through films like G.I. Joe: The Rise Of Cobra.