James Magnussen has learned the hardest way possible he's not bulletproof, but he won't be changing his outspoken ways.

The 21-year-old had his dreams of 100m freestyle Olympic gold shot down by American Nathan Adrian in Wednesday's final where he was forced to collect silver because of an agonising one-hundredth of a second.

As the pressure of sport's biggest stage took its toll, Magnussen failed to deliver under a weight of expectation, much of which he had placed on himself.

He had good reason to be confident coming to London after winning the event at last year's world championships in Shanghai and clocking a remarkable 47.10 seconds at the Australian selection trials in March.

But he made no attempt to play down the hype - declaring himself virtually unbeatable in London and warning his rivals to "brace yourselves" and speaking confidently of breaking Cesar Cielo's world record of 46.91.

While that approach set him up for a backlash from those who consider him overconfident or cocky, a shattered and humbled Magnussen insisted he would not be changing his ways.

"It's who I am as a person, it's an event that requires confidence," Magnussen said.

"... The whole way though this preparation since world championships last year, I wasn't afraid to back myself and say that I wanted to win because at the end of the day that's the truth.

"I've been honest with the media and the people of Australia the whole way along and hopefully they've responded to that honesty.

"We didn't quite get the result at the end of the day but I feel like I did my best job."

Magnussen's coach Brant Best said the swimmer's achievements and straight shooting should be celebrated not derided.

"There's always guys running around, they're bringing out the cliches and saying the set pattern. But at the end here's a kid being honest. He shouldn't be vilified or people have a go at him," Best said.

While Magnussen came to London feeling like Superman, he quickly discovered his kryptonite; the pressure of the Olympics.

After his first seamless preparation it all seemed too perfect, and then excitement turned to nerves.

He had several sleepless nights prior to the men's sprint relay final, where he was well off his best and he and his heavily favoured teammates missed out on a medal completely.

And despite an encouraging semi-final swim suggesting he had rediscovered his mojo, he couldn't find the improvement required in the final as he clocked 47.53 only to be touched out by Adrian (47.52), with Canada's Brent Hayden (47.80) in third.

"I guess having such a successful young career I just felt pretty much bullet-proof coming into this Olympics and it's very humbling," he said.

"I've got a lot more respect for guys like Michael Phelps who can come out at an Olympics and back up under pressure.

"It's just a bit of a reality check. My coach said during the week that it's a pretty tough time to learn that you're human."

Magnussen was bidding to become just the fourth Australian man, and first since Michael Wenden in 1968, to win the premier freestyle sprint at an Olympics.

He fought back tears after the race but said the experience would make him "a better swimmer and a better person".