If Dad is mentally ill, he is part of a mass psychosis. There are nearly 400 million guns in America yet the majority of Americans don't own guns, so a large, vocal minority owns a lot of guns.
Twenty is not far from average for an avid gun owner. When I was deployed to Iraq, I asked soldiers I served with how many guns they owned. I should have made a careful but the number that came up most often was sixteen. They included soldiers of every rank, enlisted and officers.
According to Pew Research, 32% of Americans own guns, just over 100 million people.
The majority, 62%, own one hand gun. That's more than 60 million people.
More than 25 million people own three to seven guns.
The top 14% own 8 to 140 guns. The average of 17 guns was in line my informal survey in Iraq. That's about 15 million people who own an average of 17 guns, or a total of 250 million guns.
If you combine all the multiple gun owners that group is more than 40 million people. Can 12% of the population be mentally ill? Not in any sane definition of mental illness.
The 40 million people who own multiple guns are part of a community in which owning many guns is normal. Nearly all of them have jobs and are part of communities. They made a choice the majority would not make, but they are not mentally ill.
Today, I talked to an 80-year-old guy who is a life-long resident of Lancaster County. He volunteers with community groups and has been part of emergency communications teams that help in disasters. He has five guns: two pistols, two shotguns and a hunting rifle. They are locked in a gun safe. He is part of a community in which five guns is not even remarkable.