"There was an actual moment where we discussed Kill 'Em All, realizing it's still vital and informative as to how we play now and where we could go moving forward. That's when we did that first Orion festival and played the whole album on one of our smaller stages as a surprise to everyone in the audience," Hammett said about Metallica's blunt-force thrash classic. "We wowed the crowd, ourselves, and came offstage thinking, 'That was fun,' doing the whole short, sharp song thing. We cycled through a handful of songs – BAM BAM BAM – rather than a few."

The Orion fest didn't last (its second iteration was in 2013 in Detroit) and lost Metallica money ("It's in our back pocket as a concept, not trashed, not dead"), but that side-stage gig was a major impetus for Metallica to show what a loose song agenda and a sharper, blunter style could do going into the next album: Hardwired.

Mention additional collaborations with pop goddess Lady Gaga and classical cellist Lang Lang, and the guitarist says that such experimentation is part of Metallica's becoming comfortable in their skin. "I do think we were guarded about who we'd let into our circle, maybe overly controlling," said Hammett. "In the last 10 years, however, we've opened ourselves up to other things, other people, more open in our thinking."

What such openness has meant for Hammett — in regard to Hardwired — was that he has approached his riffs and solos improvisationally. "Each of us look for what is unthinkable, a sonic passage that comes to you out of nowhere," he said. "What that meant for me was embracing the truly unknown, relying on pure true spontaneity and not knowing what I was going to play until I played it."

With the exception of the crash-bang epic "Spit Out the Bone," Hammett avoided preparing for the Hetfield-Ulrich-penned tracks. "That was liberating and tricky, walking into a studio without anything prepared. It takes a certain degree of confidence in your musical ability – hahaha – as well as trust in your bandmates, and they in me, that I would come up with something tasty."

Without bragging, Hammett said that his phrasing, perspective, and approach were different on Hardwired from anything else in Metallica's canon. Hammett's been part of the legacy for a long time; he joined the band in 1983, after the departure of Dave Mustaine, who went on to create Megadeth immediately following his departure.

What Metallica does hear — and eschew — after 35 years of thrashing about, is how they don't quite rock as frenetically as they used to, which is ridiculous if you're ear-deep into Hardwired's power-metal freak-outs.

"I realize when someone says as such that it is more about them, their perspective, their life, that they are projecting," Hammett said with a laugh. "Maybe they can't handle having aged 35 years. We do what we want to do — use our own instincts — and if it feels good, we hit it. We're not worried about someone outside of the room second-guessing what we do. Just the four guys inside of that room matter."

With Avenged Sevenfold and Volbeat, 6 p.m. Friday, Lincoln Financial Field, 1 Lincoln Financial Field Way. $59.50-$159.50, livenation.com.