An Oakville woman reported to Live Oak County dispatchers that the man was lying in the grass on the northbound side of Interstate 37.
Nearby the man was a red Mustang convertible with a blow-out on the right rear tire; the tire was off the rim.
When deputies arrived, they discovered the man walking around, but he could not remember what had happened.
A .22-caliber rifle was recovered from the front seat of the vehicle. The authorities assumed the wound to be self-inflicted, because no other indications of foul play were evident.
The victim, 48-year-old Von Brett Horstmann, was reportedly having health and pending legal problems.
He also had obtained head injuries in a previous motorcycle accident.
Authorities said when his family was contacted, they indicated they had been concerned that he would try to harm himself.
Halo-Flight transported him to Christus Spohn Memorial Hospital in Corpus Christi where he was listed in critical but stable condition Thursday afternoon.