Barr, a former U.S. Rep., and Loudermilk, a former Georgia state senator (R-Cassville), edged out four others to make the second round.
The winner of the runoff is expected to take the seat given there are no Democratic candidates.
Barr took 26 percent of the statewide vote, while Loudermilk took 37 percent.
Barr said his record and name recognition got him this far.
“We were able to keep the attention focused on the important issues, the substance of what’s going on in Washington and what needs to be done to correct those problems,” said Barr, who watched returns from Adventure Outdoors on South Cobb Drive in Smyrna. “I was up there and have experience to solve these things. I’ve been there and done it. That’s a message that really resonates with the people of the 11th District.”
Loudermilk gave credit to his campaign team.
“The most effective thing we did was our entire campaign team is made up of not experienced campaigners but people that hold the same vision and are dedicated to the same cause,” said Loudermilk, who watched returns at The Conservatory at Waterstone in Acworth.
“It has paid off. I’ve never seen a campaign team put in as many hours and work, time and effort as this team did.”
The seat came open when U.S. Rep. Phil Gingrey (R-Marietta) launched an unsuccessful run at the U.S. Senate.
Those knocked out of the race Tuesday were Marietta resident Tricia Pridemore, Georgia State Rep. Ed Lindsay, IT businessman Allan Levene and retired U.S. Army Col. Larry Mrozinski. Levene and Mrozinski received less than 5 percent of the vote each.
The 11th Congressional District runs from Buckhead northwest through Cobb County and into Bartow and Cherokee Counties.
Barr is perhaps the most well-known entry with eight previous years in Congress and a 2008 run for president as a Libertarian.
Earlier in the day, former Georgia Republican Party Chairman Chuck Clay predicted a runoff, but with a different name.
“I predict Loudermilk and Tricia (Pridemore) in a runoff, but Barr could be the spoiler,” he said.
“Name recognition still trumps unless it’s overwhelmingly negative, and in Barr’s case it’s not.”
Clay said there’s a deep Libertarian streak in Republican primaries that could boost Barr, but also gave credit to Loudermilk.
“Loudermilk positioned himself as a strong social conservative,” said Clay. “He also positioned himself as a traditional Chamber of Commerce candidate. I won’t say I’m surprised, but I’d say politically, he has been able to walk a fine line attracting both groups.”