The candidates took 32 percent and 18 percent support, respectively, emerging ahead of 12 other candidates to head into a Nov. 16 runoff election.
In Louisiana, all candidates are put on the same ballot, regardless of party, and if no candidate wins 50 percent or more of the vote, the top two vote-getters advance to a runoff.
Riser had long been expected to take the top spot, having gained widespread support from establishment players — including some big Washington names, like Majority Leader Eric Cantor — but second-place slot was less clear.
McAllister's win came as somewhat of a surprise, as he's a relative newcomer to politics. He poured about $400,000 of his own money into his campaign, and received a boost with the support of Phil Robertson, star of the series "Duck Dynasty."
The runoff now pits a candidate who's emphasized his experience in office and ability to get things done against one who's characterized himself as a political outsider.
Louisiana's 5th District opened up when former Rep. Rodney Alexander (R) announced his retirement in August to take a position in Gov. Bobby Jindal's (R) administration.