When starring in movies like "American Hustle" or "The Hangover," 40-year-old actor Bradley Cooper is able to stay at his normal weight around 185 pounds.

But for his Oscar-nominated role as Navy SEAL Chris Kyle in "American Sniper," Cooper had to bulk up to play the 225 pound sniper.

Kyle, who died in 2013 after being shot by a former Marine who was suffering from post-traumatic stress disorder, had 160 confirmed kills under his belt from his time as a Navy SEAL sniper in Iraq.

"I had to get to the point where I believed I was him," Cooper told Men's Health magazine. "At 185 pounds, it wouldíve been a joke. His size was such a part of who he was... Chris wasnít ripped. He wasnít sinewy. He was just a bear."

So Cooper enlisted the help of trainer Jason Walsh to help him gain more than 40 pounds in just ten weeks "with the least amount of damage to my body."

Walsh put Cooper on a rigorous diet and workout regimen. Men's Health detailed how the trainer transformed Cooper's body in such a short period of time:

They trained twice a day. In the first workout, beginning at 5 a.m., they focused on structural exercises like deadlifts and squats to build a foundation solid enough to hold the extra mass. The second workout, late in the afternoon, was more focused on traditional muscle-building exercises. Cooper needed both types of training to convincingly portray Chris Kyle.

In addition to twice daily workouts, Walsh had Cooper eating more than 5,000 calories every day. "We had to force-feed him," said Walsh. "That was the hardest part."

Cooper agreed, explaining: "It was a real shock to my body. If itís pizza and cake, thatís one thing. Putting 6,000 calories a day in your body gets old quick."

"American Sniper" writer-producer Jason Hall revealed to People magazine that Cooper "was eating about every 55 minutes" and that "he was determined to do it naturally, he didn't want to use any hormones or steroids or anything. He was just very systematic about it and took his trainer with him wherever he went." 

By the end of the 10-week training, Cooper achieved his goal of gaining 40 pounds of muscle to look like this:

Check out those guns!

Along with the massive intake of food and intense workouts, Hall told People that Cooper's preparation for the role also included twice-daily lessons with a vocal coach, as well as several hours spent watching footage of Kyle, whom he studied "over and over again." 

When it came to actually using a rifle, Cooper trained with a real Navy SEAL sniper Kevin Lacz, who served with Kyle and was a consultant on the movie. 

"He actually proved himself to be really good," added Hall. "The second day, in the morning, he went out there and was consistently hitting 800-yard targets the size of a teacup. So he took to it pretty quickly." 

After fully transforming his physical appearance and learning the new skill set, Cooper told Men's Health: "I knew this was going to be the way in to playing Chris, and it felt amazing. It's also nice to know it's possible to do it naturally, in that amount of time."

But Cooper had to shed the weight quickly for his new role in "The Elephant Man" on Broadway.

To help him lose the 40 pounds, the actor told Vanity Fair he would bring pumpkin soup to eat during rehearsals for his new play.

Cooper has since slimmed back down to his normal 185 pound weight, showing off his trimmer physique at the "American Sniper" New York premiere in December:

But the actor's transformation wasn't for nothing. "American Sniper" earned six Oscar nominations, including best picture and a best actor in a leading role nod for Cooper.

The Clint Eastwood-directed film has broken box office records, earning over $154 million worldwide on a $58.8 million production budget.

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