Page said the bullet was discharged from one of three rifles Mense was loading or unloading from his pickup truck.
Page said there is no evidence of foul play.
"All indications, at this point, are that while Mr. Mense was either placing his firearms in or removing them from his pickup, there was an accidental discharge of the firearm that fatally wounded the victim," Osage Beach police reported in a press release.
Page said detectives are waiting to receive all test results from the autopsy and will continue their investigation.
Police and emergency medical personnel were called to the business about 9:38 a.m. Saturday.
Page said several people who live and own businesses nearby were outside when they heard the gunshot and found Mense. He was pronounced dead at the scene by the Camden County coroner.
Before buying The Landing in 1983, Mense owned and operated The Depot tavern in Union for several years, according to Union Mayor Mike Livengood, a close friend. He sold The Landing to Rick Marquart in 2002.
Mense and his wife, Jane, were planning to open a restaurant, Bugsy's, in Union on the site of Rik's Steakhouse which was destroyed in a fire Dec. 29, 2008. A new building is under construction and the Menses planned to lease it for the restaurant.
"He was my best friend. He loved the community, both Union and Washington, and he was really looking forward to getting back into business," Livengood told The Missourian.
Mense was born in the Port Hudson area. His family moved to Union when he was a child and he graduated from Union High School.
Mense's brother, Jim, died March 2 of what authorities ruled was a self-inflicted gunshot wound.
