John Elliott, Victor Schwamkrug, Chris Couch, Stiles Mitchell, Shane Bertsch, Dave Schreyer, Daniel Chopra, James McLean, Stan Utley, Patrick Moore, David Branshaw and Rob Bradley are all tied for fifth place at six-under 65.
Sheehan started his round off in fine fashion as he holed a sand-wedge from 117 yards out for an eagle two at the first. He became the second player to start the round by holing out, as Jaxon Brigman did so earlier in the day.
'That was a great start,' Sheehan said. 'But when I walked off the green, the lady there said I wasn't even the first to do that today. I'm thinking, dang, I didn't even get a skin.'
He continued his quick start with birdies at the second and fourth. Sheehan then started a run of three straight birdies at the par-5 sixth. He made the turn at seven-under-par 29.
'Everything was going my way at the start,' said Sheehan, who won the Buy.Com Price Cutter Charity Championship earlier this year. 'It was kind of scary for a while. You just kind of go with it. When you start thinking, you mess up.'
Sheehan cooled off on the back. He would complete a bogey-free round with birdies on 10 and 18. He ran home an 18-footer at the last hole to claim the course record.
'I missed about a five-footer for birdie at 11,' Sheehan explained. 'If I made that one, I might have started thinking about a 59. Eleven was frustrating. I struggled a little bit on the back. I wasn't doing anything wrong, just not getting as close.'
Barranger got off to nearly as hot a start as Sheehan. Barranger birdied the first, then birdied three straight starting at the third. He polished off the front nine with back-to-back birdies starting at No. 8.
Barranger carded three consecutive birdies beginning at the 13th, but had a hiccup at the 17th when he posted a bogey six at the par-5.
'Somebody's going to shoot 59 this week,' Barranger said. 'It's out there. I think I could have done it very easily today. It's not going to surprise me to see the record get broken.'
Barranger tied the Buy.Com tour-record of 26-under-par when he won the 2001 Buy.Com Dayton Open. During Thursday's round, 98 of the 144 players in the field shot par or better.
Wes Short, Rob McKelvey, Mark Wilson, John Adams, Doug Garwood, Todd Demsey, Eric Meeks, Brian Kamm, David Dyer, Hunter Haas and Kevin Stadler are four shots of the pace at five-under.
Gore and Moore are two of the four two-time winners on the Buy.Com Tour this season. Cliff Kresge, who is tied for 28th at four-under, and Arron Oberholser, who is minus-one, are the other two-time winners.
If one of these four were to win and become a three-time winner, that person would get the automatic Battlefield Promotion to the PGA Tour. Four players have previously gained this promotion -- Chris Smith in 1997 and Heath Slocum, Chad Campbell and Pat Bates in 2001.
Full-field scores from the Gila River Classic