"He advised that he had the victim in his room and was teaching her gun safety" at the time, police said in the probable cause statement.
The girl was taken to a nearby hospital in critical condition, KTVK reported.
Glendale police Sgt. John Roth said the responding officers called the victim "the bravest, strongest girl they've ever seen."
"She was talking," he told KTVK. "She wasn't crying. She was asking questions as any 6-year-old would do."
The girl likely will undergo "multiple more surgeries and be in the hospital for months," according to the probable cause statement.
Police also said they found two more unsecured, loaded firearms – a shotgun and a revolver – in the home.
Mcinville is being held on a $50,000 cash bond and is scheduled to appear in court again May 6, KTVK reported.
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