I would consider that my wife shooting me because I missed the front door lock and rattled the keys when I got home in the dark from the trip a day early, or if I left a loaded gun on the table, meaning to put it away, when the phone rang and the next thing I heard was a loud bang and my three year old son was dead on the floor as reasonably sound reasons. You can easily check the historic record.
If you don't know what equal force is you should lock your firearms away before studying it, because it's very easy to be charged with murder.
In very simple terms, not necessarily the exact legal definition, if someone enters your property you can tell them to get out.
If they yell back at you, you can yell at them.
If they continue, and push you, you can push them to defend yourself
If they hit you with a fist, you can hit them with a fist to defend yourself
If they hit you with a baseball bat, you can hit them with a baseball bat to defend yourself
If they brandish a gun, you can brandish a gun to defend yourself
If they shoot at you, you can shoot at them to defend yourself
In order to be defending yourself they must be attacking you at the time. There was one case where some ferals drove their car into a person's yard, beat him up badly, I think robbed him and then jumped back in the car and reversed back into his house. By then he'd grabbed his rifle and fired through the back window injuring or killing a couple, and I think was charged with murder, because it was not equal force.
So be careful when you try to live in the past.