Edward Ates claims he didn't have the energy to accurately shoot Paul Duncsak and make a quick getaway.
Under cross-examination Thursday, Ates admitted he once used his revolver to shoot a snake during a road trip.
The 62-year-old was 285 pounds when Duncsak was killed in 2006.
Duncsak and Ates' daughter were involved in a bitter custody dispute after their divorce.
A defense lawyer says the prosecution theory would mean that the killer would have had to run up a set of stairs. He says Ates couldn't have done that and fired a gun accurately. He says exertion would cause his hands to shake.